Placoid Scale Research Articles

Drag reduction characteristics of the placoid scale array skin sup-ported by micro Stewart mechanism based on penalty immersed boundary method

Sharkskin performs an excellent flow drag reduction property, leading to numerous biomimetic research to obtain artificial drag reduction structures for underwater vehicles. The existing work mainly focused on replicating or simplifying the morphology of the placoid scale, however, such bionic structures cannot fully reproduce the drag reduction effects of sharks swimming in natural environments. This is because the existing research has not considered the influence of the multi-degree freedom motion characteristics of the placoid scale. Therefore, a new bionic drag reduction skin is proposed by combining micro Stewart mechanisms and placoid scales, where the micro Stewart mechanism is able to achieve the multi-degree freedom motion characteristics. The numerical results indicate that micro Stewart mechanisms inhibit the generation and development of the streamwise and spanwise vortex structures in the bionic placoid scale array, which can delay flow separation and improve the drag reduction effect of the placoid scale array. Compared with the bionic placoid scale skin and the blade groove skin without multi-degree freedom motion foundation, the new bionic skin achieves relative drag reduction efficiency of 15.78 % and 14.18 % at maximum, respectively, by reasonably adjusting the bionic skin parameters including the leg stiffness, leg damping, upper platform mass, upper platform center of mass height of the micro Stewart mechanism and the skin element distribution spacing. The research presents a novel sharkskin bionics design and reveals the mechanism of skin drag reduction to some extent.

Co-simulation of drag reduction of placoid scale oscillation driven by micro Stewart mechanism

To reduce the drag of underwater vehicles during navigation, this paper proposes a skin imbricated with bionic placoid scale based on micro-Stewart mechanism. The skin is composed of bionic shark placoid scales and Stewart structure with multi-dimensional motion characteristics, which can well simulate the multi-dimensional oscillation motion of shark scales during swimming. A co-simulation platform of computational fluid dynamics and multi-body dynamics is established to investigate the impact of oscillating parameters (heave and pitch) on the drag reduction performance of the skin. The novel skin shows a remarkable drag reduction performance, with a relative drag reduction rate over 20% (up to 33%) in the range of Re = 105 ∼ 106. It is found that the oscillation motion generated by the placoid scales can cause the fluid inside the skin to spray upward, which can increase the thickness of the fluid boundary layer, revealing the drag reduction mechanism of the skin to some extent. Moreover, the pitching motion of the placoid scale is more effective in drag reduction than the heaving motion in the condition of Re = 105. It is expected that applying this skin to underwater vehicles can achieve satisfactory drag reduction effects.

Effect of microstructure and chemical composition on cell orientation and proliferation viability of titanium alloy processed by pulse lasers

Samples with different surface microstructures and elemental composition were fabricated by pulsed lasers with different pulse duration, including the millisecond laser remelting (RE), the bioceramic coating (BC), and nanosecond laser processing (NL) samples. The morphologies, chemical elements, phases, and valences of these samples were analyzed and characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The contact angles of the samples were measured in air and under water. Their cell activity and behavior were analyzed by cell proliferation and cytoskeleton staining experiments. The results show that the hydrophilicity and aerophobicity increase after laser treatment, and the wetting behavior of underwater bubbles is strongly related to the wettability of the sample in the air. None of the samples examined in the experiment are cytotoxic. During the initial culture, the greater unevenness of the BC and NL sample surfaces hinders the initial expansion of the cells. In contrast, the cell spreading area of the ground (G) and the RE samples is larger. The inclined sidewall in the V-shaped structure weakens the energy barrier in the groove-ridge combination (NL sample), allowing cells that rarely adhere to the sidewall to adhere successfully. In addition, the orderly distribution of adhesion sites created by laser processing further promotes cell adhesion on the inclined sidewalls. When the cells have been cultured to Day 7, the NL sample forms a clear directional distribution of cells with its unique V-shaped structure mimicking the placoid scale on the shark skin surface.

Indicators of functional integrity in production animals

An ability to describe the quality of lived experience across the course of an animal’s life is of increasing interest in studies on wellbeing in farm animals. Integrity of physical functions is an important aspect of the tripartite model of wellbeing: feeling, functioning and interacting well. This review examines measures of functional integrity as potential indicators of functional (eudaimonic) wellbeing. Copious literature shows that coping with day-to-day fluctuations in the environment influences (1) periodicity of biorhythms, (2) allocation of resources to growth, defence and repair, and (3) rate of biological aging. Here we review opportunities for assessing functional integrity through analysis of (1) the dynamic status of physiological and behavioural variables, (2) the accretion of resources in production tissues (milk, eggs, body mass, offspring) and physical structures (fibre, hoof, bone, tooth, placoid scale, otolith, beak), and (3) biological age. A reduction in periodicity of biorhythms, complexity of behaviours, or uniformity of trajectories of growth and production is often associated with poorer current and future health outcomes and shorter functional longevity. These analytical approaches are grounded in statistical measures of stability in complex dynamic systems. Additionally, the cumulative effect of wear and tear on biological age can be assessed via telomere length, DNA methylation, biomarkers of allostatic load, hair cortisol, and physiological functional capacity. Timeseries of behaviour, physiology and accretion provide a richer record of functional integrity than do cumulative indicators of biological age. Indicators of integrity are needed for each phase of the animal’s life course including attainment of developmental competencies, stability of mature performance, and rate of decline with advancing age, to provide a comprehensive record across the whole of the animal’s life. The utility of measures may differ between real-time and retrospective assessments and between application to every individual versus sentinels. Further validation is required for interpretating measures of functional integrity as indicators of the functional aspect of wellbeing, together with determining their practicality and reliability before translation to commercial settings is feasible. A single type of indicator of functional wellbeing (e.g. DNA methylation) is unlikely to have utility for all life stages, species, production systems, and environments.

Placoid scales in bioluminescent sharks: Scaling their evolution using morphology and elemental composition

Elasmobranchs are characterised by the presence of placoid scales on their skin. These scales, structurally homologous to gnathostome teeth, are thought to have various ecological functions related to drag reduction, predator defense or abrasion reduction. Some scales, particularly those present in the ventral area, are also thought to be functionally involved in the transmission of bioluminescent light in deep-sea environments. In the deep parts of the oceans, elasmobranchs are mainly represented by squaliform sharks. This study compares ventral placoid scale morphology and elemental composition of more than thirty deep-sea squaliform species. Scanning Electron Microscopy and Energy Dispersive X-ray spectrometry, associated with morphometric and elemental composition measurements were used to characterise differences among species. A maximum likelihood molecular phylogeny was computed for 43 shark species incuding all known families of Squaliformes. Character mapping was based on this phylogeny to estimate ancestral character states among the squaliform lineages. Our results highlight a conserved and stereotypical elemental composition of the external layer among the examined species. Phosphorus-calcium proportion ratios (Ca/P) slightly vary from 1.8-1.9, and fluorine is typically found in the placoid scale. By contrast, there is striking variation in shape in ventral placoid scales among the investigated families. Character-mapping reconstructions indicated that the shield-shaped placoid scale morphotype is likely to be ancestral among squaliform taxa. The skin surface occupied by scales appears to be reduced in luminous clades which reflects a relationship between scale coverage and the ability to emit light. In luminous species, the placoid scale morphotypes are restricted to pavement, bristle- and spine-shaped except for the only luminescent somniosid,Zameus squamulosus, and the dalatiidMollisquama mississippiensis. These results, deriving from an unprecedented sampling, show extensive morphological diversity in placoid scale shape but little variation in elemental composition among Squaliformes.

Inspired by placoid scale to fabricate MXene derivative biomimetic structure on the improvement of interfacial compatibility, mechanical property, and fire safety of epoxy nanocomposites

• A placoid scale like biomimetic structure (PEPA-IPTS@Ti 3 C 2 T x ) was synthesized. • The compatibility between PEPA-IPTS@Ti 3 C 2 T x and EP matrix was greatly enhanced. • The tensile strength and impact strength of EP nanocomposites increased by 30.1% and 43.5%, respectively. • The PHRR and THR of EP5 showed a 47.3% and 44.1% reduction, respectively. • EP nanocomposites acquired superior toxicity reduction and smoke suppression due to multi-element catalytic effect. In recent years, developing high performance thermosetting nanocomposites has attracted wide-ranging research attention. As a rising two-dimensional material, MXene still confront the intrinsic weakness for preparing epoxy resin with superior comprehensive performance. Inspired by the overlap architecture of placoid scale, an in-situ growth technique was applied to fabricate biomimetic-structural MXene derivative, which displayed significant increase in interfacial compatibility, mechanical property, and flame retardancy of epoxy resin (EP). In contrast to pure EP, the tensile strength and impact strength of EP nanocomposites increased by 30.1% and 43.5%, respectively. Furthermore, the fire hazard of EP nanocomposites was significantly reduced. With 5 wt% of PEPA-IPTS@Ti 3 C 2 T x incorporated into EP (EP5), the peak of total heat release (THR) and total smoke production (TSP) decreased by 44.1% and 26.8%, respectively. Therefore, biomimetic structure nanohybrid (PEPA-IPTS@Ti 3 C 2 T x ) provides a facile and reliable strategy to prepare EP nanocomposites with excellent comprehensive performances.

Simulation and experimental study on drag reduction and anti-adhesion of subsoiler with bionic surface

A new subsoiler with placoid scale microstructure bionic surface was proposed which mimicked shark skin to reduce tillage resistance and soil adhesion during subsoiling cultivation. The contour curves of placoid scale microstructure on shark skin were fitted, and two kinds of bionic subsoiler with continuous and discontinuous microstructures were designed and fabricated, respectively. The effects of different bionic surfaces on tillage resistance were investigated by finite element simulation and experiment. The results indicated that the bionic subsoiler with discontinuous microstructure reduced the horizontal and vertical force by 21.3% and 24.8%, respectively. The subsoiler with discontinuous microstructure surface can prevent the adhesion between the soil and subsoiler surface more efficiently. Keywords: drag reduction, anti-adhesion, subsoiler, bionic surface, placoid scale microstructure, Finite element simulation DOI: 10.25165/j.ijabe.20221504.6531 Citation: Niu J P, Luo T Y, Xie J Q, Cai H X, Zhou Z K, Chen J, et al. Simulation and experimental study on drag reduction and anti-adhesion of subsoiler with bionic surface. Int J Agric & Biol Eng, 2022; 15(4): 57–64.

A Third Luminous Shark Family: Confirmation of Luminescence Ability for Zameus squamulosus (Squaliformes; Somniosidae).

Since recently, shark's bioluminescence has been recorded from two Squaliformes families, the Etmopteridae and Dalatiidae. Pictures of luminescence, light organ morphologies and physiology of the luminous control have been described for species of the Etmopteridae and Dalatiidae families. In 2015, a third luminous family, Somniosidae, was assumed to present a bioluminescent species, Zameus squamulosus. Up to now, confirmation of the luminous abilities of Z.squamulosus is lacking. Here, the luminescence of Z.squamulosus was in vivo recorded for the first time confirming the bioluminescence status of the third luminescent shark family. Additionally, photophore histology revealed the conservation of the light organ morphology across the luminous Squaliformes. Light transmittance analysis through the placoid scale added information on the luminescence efficiency and highlighted a new type of bioluminescent-like squamation. All these data reinforced the likelihood that the common ancestor of Dalatiidae, Etmopteridae and Somniosidae may already have been luminescent for counterillumination purpose.

A Lower Cretaceous chondrichthyan dermal denticle assemblage and its bearing on placoid scale diversity and histology

Dermal denticles of Mesozoic chondrichthyans are rarely documented, which tremendously limits the knowledge of different morphotypes during this time and for the whole clade. Here we describe an extraordinarily diverse assemblage of dermal denticles from deposits of Austria, comprising 17 different morphotypes. Based on the functional types of the denticles and their morphological similarities with both extinct and extant representatives, we attempt an order-level taxonomic attribution of the morphotypes recovered. The herein recorded occurrences of Hexanchiformes, Squaliformes, Squatiniformes, Heterodontiformes, the family Protospinacidae (Squalomorphii incertae sedis), and probably Rajiformes or Chimaeriformes increase previous tooth-based taxic diversity estimates of Valanginian cartilaginous fishes from Austria from four to at least nine orders. Additionally, we discuss the absence of denticles of cladodontomorph chondrichthyans, whose teeth were previously reported from the same deposits. Etched sections and light microscope analysis were carried out using the most abundant morphotypes with the aim of exploring the histology of chondrichthyan dermal denticles. Our results confirm previous studies in reporting a single crystallite enameloid, but we also report for the first time the presence of bundled enameloid in dermal scales, a condition so far restricted to the tooth enameloid of some chondrichthyans. Additionally, a putative new dentine type is observed, leading to the introduction of the descriptive term ‘nepedentine’ for the internal, dense, and poorly supplied dentine core by vascular channels.

Large deadfalls of the ʻginsuʼ shark Cretoxyrhina mantelli (Agassiz, 1835) (Neoselachii, Lamniformes) from the Upper Cretaceous of northeastern Italy

Cretoxyrhina mantelli was a large pelagic lamniform shark geographically widespread during the Late Cretaceous, and well known because of several nearly complete skeletons from the Western Interior Seaway of North America. Here we report 15 partial skeletons belonging to lamniform sharks from the ‘lastame’ lithozone of the Upper Cretaceous Scaglia Rossa Formation of the Lessini Mountains (northeastern Italy). Seven partial but articulated skeletons include tooth sets that allow a confident attribution to Cretoxyrhina mantelli based on dental morphologies. We review the taxonomic history of C. mantelli, evidencing that the taxon was erected by Agassiz (1835) and tracing back four of the original syntypes. Based on calcareous plankton biostratigraphy, the rock in which the Italian skeletal remains are embedded is constrained to the middle-upper Turonian. Total length estimates of the specimens suggest that the sample includes the largest specimen of Cretoxyrhina mantelli (615–650 cm estimated total length) known to date. The placoid scale morphology indicates that C. mantelli most likely was a fast swimmer with a similar ecology as the extant white shark, Carcharodon carcharias. The associated skeletal elements of the specimens represent large chondrichthyan deadfalls and the cadavers decayed on the seafloor where they remained exposed for several months, as indicated by bioerosional traces, some of which are interpreted as a product of bone-eating worm activities and other bioerosional traces with Gastrochaenolites-like structure. The Cretoxyrhina mantelli remains described herein provide new information about the ʻlastameʼ vertebrate assemblage, which seemingly was strongly dominated by chondrichthyans, especially lamniform sharks.

Microfabricated Biomimetic placoid Scale-Inspired surfaces for antifouling applications

Marine biological fouling or ‘biofouling’, the unwanted aggregation of aquatic organisms such as algae, barnacles, and marine microorganisms has been a detriment to maritime industries. Previous antifouling strategies included the use of toxic, biocide-containing paints; however, as humanity strives to lessen its environmental impact, a shift towards ecological deterrents which draw inspiration from nature are starting to be developed. In this work, the manipulation of the surface topography of nontoxic polydimethylsiloxane elastomer (PDMSe) was performed in order to mimic natural-occurring antifouling surfaces like those found on shark skin by means of a micro-molding technique using etched silicon molds. Previous polymer-based antifouling patterns which draw influence from shark skin have been successful in mitigating microorganism settlement; however, they lack a degree of biological accuracy as the features are of constant height. Our novel designs utilize various microfabrication techniques and micro-molding to generate placoid scale patterns with an engineered height gradient to deter organism settlement. Surface durability studies showed that the patterns can effectively keep their integrity under external sliding motion. Significant decreases in Escherichia coli (E. coli) settlement up to 75% were observed when measuring the effectiveness of pristine patterns, and up to 56% when patterns underwent extreme mechanical wear.

DEM simulation of bionic subsoilers (tillage depth >40 cm) with drag reduction and lower soil disturbance characteristics

Successive years of mechanical operations and conservation tillage in Northeast China have made subsoiling necessary, and the tillage depth can be no less than 40 cm. In this paper, a bionic design method was used to reduce subsoiler energy consumption and soil disturbance. The bionic structural elements, i.e., triangular prism (BTP) and partial circular column (BPCC), were inspired by the placoid scale rib structure of shark skin, which has low drag. These elements were then applied to the subsoiler to reduce energy consumption. Six types of bionic subsoilers were designed. Discrete element modeling (DEM) was used to simulate and analyze the interactions of the bionic subsoilers and an ordinary subsoiler (O-S) with the soil. The results showed that bionic subsoilers with a shank and BTP in the horizontal direction of motion (S-T-H) and tines with the BTP parallel to the centerline (T-T) had lower draft requirements and energy consumption than the other designs. The draft requirements and energy consumption of S-T-H subsoilers with different height-to-lateral-rib-spacing (h/s) ratios were then compared. The subsoiler with a bionic element h/s of 0.57 (S-T-H-0.57) had a lower draft requirement (1292.59 N) and a lower total energy requirement (23.48 J) than the other designs. A soil disturbance analysis demonstrated that S-T-H-0.57-T-T (bionic elements arranged in both the shank and tine) provided superior benefits in terms of root growth and improved crop stress resistance. The result is consistent with the mechanical analysis of the riblet, which will be helpful for designing new subsoilers with reduced drag and soil disturbance.

O-40 楯鱗化石からみた西南日本中期中新世の海洋環境(6. 中国地方の第三系-層序と古環境-,口頭発表,一般発表)

O-40 楯鱗化石からみた西南日本中期中新世の海洋環境(6. 中国地方の第三系-層序と古環境-,口頭発表,一般発表)

Denticle-embedded ampullary organs in a Cretaceous shark provide unique insight into the evolution of elasmobranch electroreceptors.

Here, we report a novel type of dermal denticle (or placoid scale), unknown among both living and fossil chondrichthyan fishes, in a Cretaceous lamniform shark. By their morphology and location, these dermal denticles, grouped into clusters in the cephalic region, appear to have been directly associated with the electrosensory ampullary system. These denticles have a relatively enlarged (∼350 μm in diameter), ornamented crown with a small (∼100 μm) asterisk- or cross-shaped central perforation connected to a multi-alveolate internal cavity. The formation of such a complex structure can be explained by the annular coalescence and fusion, around an ampullary vesicle, of several developmental units still at papillary stage (i.e. before mineralization), leading to a single denticle embedding an alveolar ampulla devoid of canal. This differs from larger typical ampullae of Lorenzini with a well-developed canal opening in a pore of the skin and may represent another adaptive response to low skin resistance. Since it has been recently demonstrated that ampullary organs arise from lateral line placodes in chondrichthyans, this highly specialized type of dermal denticle (most likely non-deciduous) may be derived from the modified placoid scales covering the superficial neuromasts (pit organs) of the mechanosensory lateral line system of many modern sharks.

Scale morphology and flexibility in the shortfin mako Isurus oxyrinchus and the blacktip shark Carcharhinus limbatus

We quantified placoid scale morphology and flexibility in the shortfin mako Isurus oxyrinchus and the blacktip shark Carcharhinus limbatus. The shortfin mako shark has shorter scales than the blacktip shark. The majority of the shortfin mako shark scales have three longitudinal riblets with narrow spacing and shallow grooves. In comparison, the blacktip shark scales have five to seven longitudinal riblets with wider spacing and deeper grooves. Manual manipulation of the scales at 16 regions on the body and fins revealed a range of scale flexibility, from regions of nonerectable scales such as on the leading edge of the fins to highly erectable scales along the flank of the shortfin mako shark body. The flank scales of the shortfin mako shark can be erected to a greater angle than the flank scales of the blacktip shark. The shortfin mako shark has a region of highly flexible scales on the lateral flank that can be erected to at least 50°. The scales of the two species are anchored in the stratum laxum of the dermis. The attachment fibers of the scales in both species appear to be almost exclusively collagen, with elastin fibers visible in the stratum laxum of both species. The most erectable scales of the shortfin mako shark have long crowns and relatively short bases that are wider than long. The combination of a long crown length to short base length facilitates pivoting of the scales. Erection of flank scales and resulting drag reduction is hypothesized to be passively driven by localized flow patterns over the skin.

Attachment of Dermophthirius carcharhini (Monogenea: Microbothriidae) to the Galapagos Shark Carcharhinus galapagensis

Attachment of Dermophthirius carcharhini (Monogenea: Microbothriidae) to the Galapagos Shark Carcharhinus galapagensis

Functions of Scales and Photophores in Mesopelagic Luminescent Sharks

AbstractIn sharks bioluminescence is only known from the family Squalidae. It evolved independently in two out of six squalid subfamilies, Dalatiinae and Etmopterinae. The distribution of photophores was mapped in several species. It is suggested that in the Dalatiinae, which do not school, but migrate vertically, luminescence serves as ventral countershading. The Etmopterinae school and feed close to the bottom. Their luminescence is an aid in schooling. Four different placoid scale patterns are found in luminescent sharks and they allow to accommodation the photophores in the skin.

Hypoprion bigelowi, a Synonym of Carcharhinus signatus (Pisces: Carcharhinidae), with a Description of Ontogenetic Heterodonty in This Species and Notes on Its Natural History

The upper jaw tooth morphology of the night shark, Carcharhinus signatus, undergoes marked ontogenetic change, with symphyseal serrations increasing in both size and number relative to the commissural edge of each tooth. Similarly, placoid scale keel number increases with increasing total length. Since the eastern Atlantic Hypoprion bigelowi is differentiated from C. signatus on the basis of these two characters, H. bigelowi represents an ontogenetic stage, and therefore is a synonym of C. signatus. In the western North Atlantic, this species occurs commonly in the Florida Current and in the Gulf Stream off the east coast of the United States and may range as far north as the middle Atlantic Bight on the outer continental shelf during the warmer months. Fresh specimens for this study were caught near the surface at dawn and contained fish and cephalopod remains. Animal associates included Remora remora, Pandarus smithii and Aega sp. Anomalous dentition is noted in two of the specimens.

Chemical nature of proteins in the placoid scale of the blue shark, Prionace glauca L

Powdered placoid scale was fractionated with differential density-flotation into three fractions, Frs L (density, 1.7 to 2.3), M (more than 2.3 to 2.6) and H (more than 2.6), in a proportion of 11:2:1. Fr L was regarded as dentine and Fr H as enameloid. Protein content of dentine was 16.7 per cent and of enameloid 5.6 per cent. Collagen was the major protein in both. After demineralization with 0.5 M EDTA, collagen was extracted with 0.5 M acetic acid at 75 °C. Proteins that were soluble in 0.5 M EDTA and in 10 mM NaOH after the extraction of collagen, A, were fractionated into four groups of proteins, and three were phosphoproteins of varying phosphate contents (about 4, 8 and 40 moles per 100 amino acids). Other noncollagenous proteins of low phosphate contents (2 moles) were divided by solubility differences into 6 M-guanidine-soluble (B), thiol-guanidine-soluble (C) and thiol-guanidine-insoluble proteins (D). The proportion of collagen: A: B: C: D was 66:15:3:4:12 in dentine and 44:19:4:3:30 in enameloid. The amounts of protein groups in A also differed notably in the two. The chemical compositions of phosphoproteins in A resembled those reported for phosphoproteins of mammalian dentine. Compositional patterns of B, C and D were similar to the gross amino acid composition of mature mammalian enamel and close to those reported for peptides released from the amino-terminal region (B and D) and carboxy-terminal region (C) of procollagen.

The phylogeny of the chimaeroids

The purpose of this paper is to examine the evidence afforded by the chimaeroids and fossil holocephalans on the origin and evolution of the Holocephali, and, in particular, on the hypothesis recently proposed by Ørvig of a relationship between chimaeroids and ptyctodont arthrodires. Members of all the fossil holocephalan groups which are known by moderately complete specimens except the petalodonts and edestids are described, particular attention being paid to the histology of the dermal skeleton. Three species from the English Lower Lias,Squaloraja polyspondylaAgassiz,Myriacanthus paradoxusAgassiz andMetopacanthus granulatus(Agassiz) (a species previously placed inMyriacanthus) are described in detail. From the British Lower CarboniferousDeltoptychius armigerus(Traquair), previously placed inOracanthus, is described in detail and two new species,Deltoptychius moythomasisp.nov. andMenaspacanthus armaghensisgen. and sp.nov., are described on fragmentary material. The genusOracanthusAgassiz is reviewed: it is shown that the type species is probably acanthodian but that many species have been wrongly included in the genus. There is found to be strong evidence that the chimaeroids are derived, through the Jurassic myriacanthids and allied forms, from a Palaeozoic group which includes the PermianMenaspisand whose most primitive known genus isDeltoptychius. The most striking feature ofDeltoptychiusis the dermal armour of dentine-like tissue on the head which includes a complete head shield bearing a pair of sensory canals. This armour is of a peculiar type which is not homologous with the armour of the arthrodires, and appears to be confined to the Chimaeriformes. It is suggested thatHelodusis not directly related to the Chimaeriformes. These conclusions are embodied in a new classification of the Holocephali in whichHelodusis made the type of a new order Helodontiformes and in which the Chimaeriformes is divided into four sub-orders, Chimaeroidei, Squalorajoidei, Myriacanthoidei and Menaspoidei, with new families being made in the last two sub-orders. On the basis of the information obtained from the systematic part, the origin and evolution of the various characters of chimaeroids are discussed. The ethmoid canal appears to be a recent acquisition which occurs only in Chimaeroidei. The tritors on chimaeroid tooth plates are another recent adaptation. There is no convincing evidence that the teeth of Holocephali were primitively numerous, as they are inHelodus, petalodonts and edestids: this ‘selachian’ type of dentition is quite possibly secondarily derived from a 1chimaeroid ’ type of dentition. The dorsal fin spine of the chimaeroids appears to be a recent adaptation which first appears in the myriacanthoids and has arisen independently, being unrelated to the fin spine ofHelodus. A series of changes has taken place in the structure of the fin spine in Chimaeriformes which is similar to the changes which have occurred in the fin spines of sharks. The scales ofDeltoptychiusare cyclomorial: the change from a cyclomorial to a placoid (synchronomorial) scale has taken place independently among elasmobranchs at least three times: in the selachians, in the edestids, and in the Chimaeriformes. The scales ofHelodusare synchronomorial: this is the only Palaeozoic elasmobranch in which this condition has yet been found. The crescentic calcifications surrounding the sensory canals of chimaeroids are shown to be modified scales. The pre-pelvic tenacula of male chimaeroids have evolved from groups of enlarged scales without skeletal support which occur in myriacanthoids. It is suggested that the frontal clasper of male chimaeroids has evolved from paired structures, defensive in function, which occurred in both sexes in Palaeozoic Chimaeriformes. The dermal armour of Chimaeriformes is formed by fusion of scales. It has undergone a series of changes in histological structure which is similar to that undergone by the dorsal fin spine, but which took place much earlier in time. The evidence for a relationship between Holocephali and the selachians or arthrodires is reviewed. It is concluded that there is evidence of relationship between holocephalans and arthrodires, but that the resemblances between the two groups are in general features only. Mainly because of the structure of the exoskeleton inDeltoptychius, a relationship with the ptyctodonts is very unlikely. The holocephalans seem to be closest to the most primitive arthrodires (rhenanids and stensioellids), suggesting that the two groups share a common ancestor but are not directly related.