Lamina Orbitonasalis Research Articles

Chondrenchelys problematica(Traquair, 1888) redescribed: a Lower Carboniferous, eel-like holocephalan from Scotland

ABSTRACTChondrenchelys problematica, from the Viséan (Holkerian) of Scotland, is the earliest holocephalan known from extensive cranial and postcranial material. Here, we provide a comprehensive new description of this taxon using three new specimens, in which we observe many morphological features for the first time. Much of the cranial morphology is closer to that of living chimaeroid holocephalans than was previously appreciated. For this reason, we provide original figures illustrating the chondrocranium of a hatchlingCallorhinchus miliidemonstrating these similarities. InChondrenchelys, although the jaw articulation is positioned at the posterior margin of the orbit, the high-walled lamina orbitonasalis and densely-mineralised antorbital crest provide evidence for forward rotation of the jaw adductor musculature. Preserved foramina for the efferent superficial ophthalmic nerves show that the sensory organs on the rostrum were enervated in a manner similar to modern sharks, with the ophthalmic nerves not enclosed in an ethmoid canal, as in modern holocephalans. The conjunction of numerous distinctly holocephalan features with those that are otherwise general to Chondrichthyes demonstrates a decoupling of several of the structural conditions that characterise the distinctive morphological complex of the extant holocephalan skull. The anguiliform postcranium is more elongate than previously reconstructed, and it is now clear that the axial skeleton extended beyond the posterior extremity of the elongate dorsal fin. Morphological characters are reviewed with a view to further phylogenetic analyses. We recommend using the appearance ofChondrenchelysat 336.5 Ma as a hard minimum age for the last common ancestor of elasmobranchs and chimaeroids, because of its secure association with other holocephalans, and current uncertainties concerning elasmobranch stem lineage membership.

Skeletal morphology and development of the olfactory region of Spea (Anura: Scaphiopodidae).

The nasal capsules of anurans are formed by an intricate set of sac-like cavities that house the olfactory organ and constitute the beginning of the respiratory system. In tadpoles, nasal capsules do not have a respiratory function, but each is composed of a single soft tissue cavity lined with olfactory epithelium. Our study has revealed that in Spea the nasal cartilages and septomaxillae are de novo adult structures that form dorsal to the larval skeleton of the ethmoid region. The only element of the adult nasal capsule that is partially derived from the larval skeleton is the solum nasi. Development of the nasal skeleton begins at about Gosner Stage 31, with chondrification of the septum nasi and lamina orbitonasalis. The alary cartilage and superior prenasal cartilage are the first of the anterior nasal cartilages to chondrify at Gosner Stage 37. By Gosner Stages 40/41, the ethmoid region is composed of the larval structures ventrally and the adult structures dorsally. By Stage 44, the larval structures have eroded. The adult nasal capsule is characterized by: (1) a septum nasi that projects ventrally beyond the plane of the nasal floor; (2) a paranasal commissure that forms the ventral margin of the fenestra nasolateralis; and (3) a large skeletal support for the eminentia olfactoria formed by the nasal floor and vomer. The timing of chondrification of the anterior nasal cartilages and the development of the postnasal wall, inferior prenasal cartilage, fenestra nasolateralis, and paranasal commissure are discussed and compared with those of other anuran species. This study also includes a discussion of the morphology of the skeletal support for the eminentia olfactoria, a structure best developed in distinctly ground-dwelling frogs such as spadefoot toads. Finally, we propose a more precise restriction of the terminology that is used to designate the posterior structures of the olfactory region of anurans.

Anuran postnasal wall homology: an experimental extirpation study.

The nasal placode was extirpated unilaterally in Gosner stage 18-20 embryos of Rana sylvatica, R. palustris and R. pipiens, in order to test alternative proposed schemes of homology for the ethmoidal attachment of the palatoquadrate in anurans and urodeles. Absence of the nasal sac has no pronounced effect on the formation of larval chondrocranial structures. In contrast, in metamorphosed animals the lamina orbitonasalis and inferior prenasal process are the only nasal capsule structures present on the operated side. The medial nasal branch of the deep ophthalmic nerve passes forward over the dorsal surface of the lamina orbitonasalis, rather than through an orbitonasal foramen. Comparison with previous experimental work on urodeles supports the traditional homology of the anuran lamina orbitonasalis with the antorbital process of urodeles and other vertebrates.

Early development of the cephalic skeleton in the turbot

At hatching Scophthalmus maximus shows no cartilaginous and no bony structure. Meckeľs cartilages appear when the fry are 1 day old, followed on day 2, by formation of the trabecular bars, fused at the outset to form a trabecula communis. Concurrently, the palatoquadrates complete the mandibular arch, and the first two pairs of ceratobranchials, associated with a pair of hyoid bars, form the beginnings of the hyobranchial system. By day 3, the parachordals have fused with the trabecular bars, the hyosymplectics have linked to the hyoid bars by interhyals, and the first four pairs of ceratobranchials have appeared. The first bony structures appear: the preoperculars. On day 8, the frontals develop above the orbits and the maxillaries and dentaries appear. On day 10, the primordia of the taeniae marginales appear, the palatoquadrates bear a pterygoid process, and to the branchial basket have been added the fifth pair of ceratobranchials and the four pairs of epibranchials. On day 12, both pairs of posterior pharyngobranchials are present. The premaxillaries develop in front of the maxillaires, and retroarticulars and the angulars complete the lower jaws. On day 13, a thin parasphenoid contributes to the floor of the neurocranium, and ectopterygoids and entopterygoids to the splanchnocranium. The set of opercular bones is complete. On day 15, the tectum synoticum closes the braincase posteriorly. The splanchnocranium possesses a basihyal and the pharyngobranchials of the first epibranchials. On day 18, the tectum posterius completes the dome of the braincase. The rear end and lateral walls of the skull are formed by the basioccipital, the exoccipitals, the pterotics, and the parietals. The suspensorium is nearly complete. From day 10, the first resorptions begin in parallel with the construction of the chondrocranium. Meckeľs cartilages each split in two, then the posterior part of the trabecular bars disappears. On day 23, the right taenia marginalis separates from the lamina orbitonasalis and curves towards the centre. Simultaneously, the right eye begins its migration to the left. This is the only metamorphosis-linked asymmetry to appear during the development of the chondrocranium. On day 25, many more bony structures appear, a characteristic of this stage: the nasals, lateral ethmoids, mesethmoid, sphenotics, prootics, pleurosphenoids, epiotics, and supraoccipital. From this stage on, the bony structures continue to develop, while the front of the neurocranium and the jaws undergo a deep remodelling due to metamorphosis. The left taenia marginalis does not appear reduced until day 29. By day 45, there remain only a few small elements of the cartilaginous skull.

Early development of the cephalic skeleton in the turbot

At hatching Scophthalmus maximus shows no cartilaginous and no bony structure. Meckeľs cartilages appear when the fry are 1 day old, followed on day 2, by formation of the trabecular bars, fused at the outset to form a trabecula communis. Concurrently, the palatoquadrates complete the mandibular arch, and the first two pairs of ceratobranchials, associated with a pair of hyoid bars, form the beginnings of the hyobranchial system. By day 3, the parachordals have fused with the trabecular bars, the hyosymplectics have linked to the hyoid bars by interhyals, and the first four pairs of ceratobranchials have appeared. The first bony structures appear: the preoperculars. On day 8, the frontals develop above the orbits and the maxillaries and dentaries appear. On day 10, the primordia of the taeniae marginales appear, the palatoquadrates bear a pterygoid process, and to the branchial basket have been added the fifth pair of ceratobranchials and the four pairs of epibranchials. On day 12, both pairs of posterior pharyngobranchials are present. The premaxillaries develop in front of the maxillaires, and retroarticulars and the angulars complete the lower jaws. On day 13, a thin parasphenoid contributes to the floor of the neurocranium, and ectopterygoids and entopterygoids to the splanchnocranium. The set of opercular bones is complete. On day 15, the tectum synoticum closes the braincase posteriorly. The splanchnocranium possesses a basihyal and the pharyngobranchials of the first epibranchials. On day 18, the tectum posterius completes the dome of the braincase. The rear end and lateral walls of the skull are formed by the basioccipital, the exoccipitals, the pterotics, and the parietals. The suspensorium is nearly complete. From day 10, the first resorptions begin in parallel with the construction of the chondrocranium. Meckeľs cartilages each split in two, then the posterior part of the trabecular bars disappears. On day 23, the right taenia marginalis separates from the lamina orbitonasalis and curves towards the centre. Simultaneously, the right eye begins its migration to the left. This is the only metamorphosis‐linked asymmetry to appear during the development of the chondrocranium. On day 25, many more bony structures appear, a characteristic of this stage: the nasals, lateral ethmoids, mesethmoid, sphenotics, prootics, pleurosphenoids, epiotics, and supraoccipital. From this stage on, the bony structures continue to develop, while the front of the neurocranium and the jaws undergo a deep remodelling due to metamorphosis. The left taenia marginalis does not appear reduced until day 29. By day 45, there remain only a few small elements of the cartilaginous skull.

Origin and evolution of the anuran postnasal wall and adjacent parts of the palatoquadrate

In early anuran larvae the posterior wall of the nasal capsule is not developed. The olfactory nerves are exposed dorsally. Later paired, transversely located elements roof these nerves. They fuse and, later, produce, by downward growth, the walls of the olfactory canals. Laterally, the first functional partition between the future nasal capsule and orbit is the commissura quadratocranialis anterior. The ophthalmicus nerve runs across the dorsal surface of the commissura. In the course of further development, a new element (called the lamina orbitonasalis), arises anterolaterally to the commissura; consequently, the above mentioned nerve is enclosed in a canal betweeo the lamina orbitonasalis and the braincase. Meanwhile, the commissura becomes detached from the braincase and fixed to the lamina. Later another element, the epipraemandibulare, arises anterodorsally to the lamina; it encloses a branch of the ophthalmicus nerve in a canal. On the lateral end of the lamina orbitonasalis a further element aris...

Development and morphology of rostral cartilages in batoid fishes (Chondrichthyes: Batoidea), with comments on homology within vertebrates

Development and morphology of rostral cartilages in batoid fishes (Chondrichthyes: Batoidea), with comments on homology within vertebrates

Development of the chondrocranium of the shallow-water Cape hake Merluccius capensis (Cast.). Part 1: Neurocranium

The development of the neurocranium of Merluccius capensis, from the earliest identifiable stage, is described. The anterior mesenchyme gives rise to the rostral cartilage, the lamina orb ito nasalis and probably also to the planum ethmoidale. There is no true ethmopalatine articulation, only a rostropalatine articulation. The ethmopalatine articulation is represented by the preorbital ligaments. A cartilago supraentethmoidalis develops, but no processus ethmoideus. A preoptic root is absent and only a foramen olfactorium advehens is delimited. A lateral commissure and prootic process, and consequently a trigeminofacialis chamber, are absent. Both the auditory capsules and the occipital arches take part in the development of the tectum synoticum. The tectum posterius does not develop separately, but as a posterior extension of the tectum synoticum.

Role of the neural crest in development of the cartilaginous cranial and visceral skeleton of the medaka, Oryzias latipes (Teleostei).

Neural crestectomies were performed on neurula stage medaka embryos to remove neural crest with tungsten needles from one of five anteriorly located zones. The embryos were allowed to develop to stage 35 (immediately posthatching) larvae, then cleared and stained for cartilage. An analysis of changes to the head skeletons indicated that most of the anterior neurocranium and the entire viscerocranium received neural crest contributions during development. The elements involved included; the lamina orbitonasalis of the nasal capsule, the trabeculae, Meckels' cartilage and the quadrate of the lower jaw, the pterygoid process, the orbital cartilages and the epiphyseals of the neurocranial roof, as well as all the elements of the hyoid and branchial arches. By further analysis of only those neural crest ablations which produced alterations to the head skeleton, the neural crest cells which contributed to the development of each element were mapped. They originated principally, from one of three regions; the mesencephalon (second most anterior zone removed, number II), the preotic rhombencephalon (zone III), or the postotic rhombencephalon (zone IV). Neural crest from the level of the prosencephalon (zone I) was not chondrogenic nor was neural crest from the fifth region (zone V) which extended beyond the 5th to about the 8th or 10th somite and marked the anterior end of trunk neural crest. The data are discussed and are found to be consistent with the results from other vertebrates and support the central role of the neural crest in the development and evolution of the vertebrate head skeleton.

The development of the nasal capsule of the silver carpHypophthalmichthys molitrix(Valenciennes)

The development of the nasal capsule, including the procartilaginous as well as some mesenchymatous developmental stages, is described in the Chinese silver carp, Hypophthalmichthys molitrix (Val.). In the nasal capsule a primitive feature, namely the presence of a complete paraphysial bridge was observed. The composition and possible origin of the ethmoidal plate, nasal septum and lamina orbitonasalis in fishes are discussed.

THE DEVELOPMENT OF THE CHONDROCRANIUM IN THE SEAHORSE, HIPPOCAMPUS [LOPHOBRANCHII

SUMMARY 1 Three stages in the development of the chondrocranium of Hippocampus are studied. In the last stage it is fully formed. 2 The ethmoid plate has an upward and backward bend in the 9mm. stage of Hippocampus. This flexure gradually straightens and the normal horizontal condition is assumed in the fully formed chrondrocranium. 3 The lamina orbitonasalis and the nasal septum exhibit their independent origin from the ethmoid plate by retaining their identity even after fusing with the ethmoid plate. 4 The orbital cartilages are reduced and are in the form of a pair of short processes projecting back from the laminae orbitonasales. 5 The trabecula communis becomes disconnected from the basal plate in the 12mm. stage. It is present in the fully formed chondrocranium as a small posterior process of the ethmoid plate. Hence the ethmoid region of the chondrocranium is independent of the occipito-auditory region. 6 A pila lateralis develops in the 18mm. stage enclosing the trigeminal nerve. In the earlier stages there is only a groove, the incisivum pro-oticum. 7 A lateral commissure is completely developed forming the ventrolateral wall to the trigemino-facialis chamber. 8 There is a minute basicapsular fenestra in stage 1. It becomes confluent with the metotic foramen in later stages. 9 The otic capsule does not show external prominences of the semicircular canals. There are no complete cartilaginous septa separating the semicircular canals. 10 There is no tectum synoticum. A slender tectum posterius connects the two occipital processes. 11 A vestigial taenia tecti medialis is the only roofing cartilage in the chondrocranium. 12 The pterygoid process of the quadrate is vertical in the 9mm. stage and it assumes a horizontal position in the 18mm. stage. The process is continuous with the body of the quadrate by means of a thin cartilaginous extension in early stages. The two parts become separated as the ethmoid plate is straightened. The pterygoid process which is at first fused with the ethmoid plate articulates later by means of a distinct rostropalatine process with the pre-ethmoid cornu conforming to the “ acrartete ” condition of Swinnerton's description. 13 The quadrate in the 9mm. stage is fused with Meckel's cartilage and separates from it later. 14 Meckel's cartilages are obliquely situated in the early stages and undergo a rotation of about 90 degrees to assume a horizontal condition along with the straightening of the ethmoid plate. 15 The hyosymplectic cartilage is elongated and extends to Meckel's cartilage and lies below the quadrate in early stages. In later stages the anterior end of the hyosymplectic lies behind the quadrate. The hyomandibula articulates with the otic capsule in front of the region of the lateral semicircular canal. A spheno-pterotic ridge overhangs the articular facet. 16 The copula communis in the fully formed stage does not extend beyond the first branchial arch. There are no basibranchial elements in the branchial arches. 17 The hypohyal and ceratohyal are distinct cartilages and undergo a change in position and size during development. 18 A well developed rostral cartilage is present on the dorsal surface of the distal end of the ethmoid plate from the early stages: It also moves with the ethmoid plate to assume an anterior position. 19 Infralabial cartilages are present in the form of reduced dense cellular masses. 20 Sclerotic cartilages are present in the optic capsules.

THE DEVELOPMENT OF THE CHONDROCRANIUM IN OPHICEPHALUS*.

Summary. 1 Eight stages in the development of skull elements, commencing from the first day to twenty-five days after hatching, have been studied. 2 The mesenchymatous plate in contact with the ectoderm on either side of hypophysis is seen in stage 1 (2.75 mm.). 3 The trabeculae and parachordals arise independently in stage 2 (4.0 mm.). 4 The trabeculae are joined to the parachordals, leaving a wide hypophysial fenestra in stage 3 (4.0 mm.). The visceral arches chondrify and the first two branchial arches appear in this stage. The basibranchials have fused to form a copula communis. 5 In stage 4 (4.5 mm.) the rudiments of the occipital arches are noticed, A basicapsular fenestra is seen. The pterygoid process is noticed as an independent chondrification as in Clupea, and the quadrate is fused with the hyomandibula, as also in Clupea. All the five branchial arches have developed in Ophicephalus. The fifth branchial arch extends to a few sections. 6 In stage 5 (5.0 mm.) the pharyngobranchials have developed in connection with the first three branchial arches. The metotic foramen is delimited by the fusion of the occipital arch with the auditory capsule posteriorly. Meckel's cartilages have fused anteriorly at the symphysis. 7 In stage 6 (5.0 mm.) the pterygoid process is connected with the ethmoid plate by connective tissue. 8 In stage 7 (6.0 mm.) the pterygoid process extends posteriorly and is fused with the quadrate. An interhyal fused with the hyomandibula is noticed to be in contact with the ceratohyal. 9 In the fully formed chondrocranium, stage 8 (24.0 mm.), the rostral cartilage is developed and is not fused with any part of the ethmoid plate. The lamina orbitonasalis develops on each side of the middle region of the ethmoid plate. There is a small process directed posteriorly at the point where the lamina orbitonasalis has fused with the nasal septum, representing the anterior portion of orbital cartilage. The ophthalmic branch of the trigeminal nerve passes in a groove dorsal to the lamina orbitonasalis. The trabeeulae disorganize and are represented as a small extension posterior to the trabecula communis. The hypophysial fenestra is obliterated. The two orbital cartilages are united in the middle region of the chondrocranium by an epiphysial bar. A small taenia tecti medialis extends posteriorly from the middle region of the epiphysial bar. The trigemino-facialis chamber has no complete lateral commissure. The basicapsular fenestra and the metotic foramen are separated by the basivestibular commissure. The two auditory capsules are joined by a tectum synoticum. There is no tectum posterius. There are three prominences externally on the auditory capsules for the accommodation of the three semicircular canals. The hyomandibular branch of facial nerve passes in a foramen in the hyomandibula. The quadrate is fused with the hyomandibula. The hyomandibula is connected with the ceratohyal by a stylohyal. The pterygoid process is discontinuous with the quadrate. Meckel's cartilage articulates with the quadrate and there is a retroarticular process of Meckel's cartilage. In the branchial arches the basihyal is continuous with the copula communis. The ceratohyal shows a stout hypohyal in close contact with the basihyal. There is a foramen in the hypohyal for the hyoidean artery, recorded only in Solea. The first four branchial arches show differentiation into the hypo-, the cerato-, the epi-and the pharyngobranchials. The fifth branchial arch is small and consists of a single element—the ceratobranchial. The pharyngobranchials of the third and fourth arches are fused. 10 There are two small rods in connection with the second branchial arch ventrally on either side of the copula communis. I have called these the sub-copular rods. The homology of these rods is not clear. 11 In the chondrocranium the ethmoid region is completely separated from the auditory region by the discontinuity of trabeculae, the orbital cartilages and the pterygoid processes. 12 The chondrocranium is platytrabic, as no interorbital septum is noticed.

The metamorphosis of the jaws and their muscles in the Toad, Bufo regularis Reuss, correlated with the changes in the animal's feeding habits

SUMMARY.In the tadpoles of Bufo regularis it has been found that: A commissura quadrato‐cranialis anterior and an ascending process are present; both of these structures are of common occurrence in all other Salientia and help in joining the quadrate to the skull. There is no larval otic process (a primitive character), but instead there is a fibrous ligament, the quadrato‐otic ligament. The processus muscularis quadrati is attached to the lamina orbitonasalis by a bridge of cartilage which is here called the “commissura quadrato‐orbitalis”. The infrarostral cartilage and Meckel's cartilage are connected by a cartilaginous bar, which is here called the “commissura intramandibularis”. The suprarostral cartilage consists of a median and two lateral lobes. This is a primitive character. There is neither a M. l. m. anterior lateralis (as in Ascaphus truei), nor a M. l. m. subexternus (as in Bufo bufo). The M. l. m. externus consists of two separate parts, anterior and posterior, the former is inserted on the lower jaw, the latter on the suprarostral cartilage. The M. mandibulo‐labialis is inserted on the lower lip only, and has no connection to the upper lip. The important changes which take place during metamorphosis are: All three larval processes joining the quadrate to the skull break down and disappear. The otic process develops exclusively from the posterior half of the quadrate. The pseudo‐basal process develops as an outgrowth from the anteroventral surface of the auditory capsule. A quadrate bone develops on the articular part of the quadrate cartilage and fuses with the quadratojugal, which develops on the ligamentum cornu quadratum laterale. The M. mandibulo‐labialis fuses with the M. intermandibularis posterior. The two parts of the M. l. m. externus fuse with each other. The MM. orbitohyoideus, suspensoriohyoideus and the suspensorioquadrato‐, and hyo‐angularis fuse and give rise to the M. depressor mandibulae. The MM. l. m. posterior superficialis and profundus remain separate throughout life. The M. l. m. anterior lateralis is differentiated from the M. l. m. anterior articularis late during metamorphosis. The jaw muscles show a drop in their absolute lengths as well as in volume during metamorphosis, after which they begin to grow again. The lower jaw keeps on increasing in length until the adult condition is attained.

The morphology of the chondrocranium of Gymnarchus niloticus.

SUMMARY. The chondrocranium of two larval stages of Gymnarchus niloticus—one 15-day old, the other of an unknown but of slightly younger stage—have been studied. The morphological relations of the arteries, veins and nerves of the head have been described in detail, and the results have necessitated a rectification of some of the statements made by Assheton. The cartilage of the chondrocranium is greatly developed but conform to the general Teleostean type with certain peculiarities of its own. The larval skull is platytrabic. The auditory capsule is greatly developed and contains a modified ear and a sac-like extension of the air-bladder. The internal cavity of the capsule is divided by a partition which is partly ossified. In the lateral wall of the capsule is a vacuity covered by a membrane lateral to which extends the expanded portion of the temporal and supratemporal latero-sensory canals. There is an extensively developed orbital cartilage with pre-optic and metoptic roots and a pila antotica secundaria. The anterior and posterior myodomes are absent. All the eye muscles are collected in a fibrous orbital capsule. The internal carotid foramen is separated from the hypophyseal fenestra. There is no pro-otic bridge, There is a well-developed trigemino-facialis chamber. A lateral commissure is present. Nasal capsules are practically absent except for a broad and massive internasal septum and a rudimentary lamina orbitonasalis. The pterygoquadrate is in a pro-cartilagenous fusion with the hyomandibula. Neither otic, nor basal, nor basi-trabecular articulations are present. The larval jaw suspension is methyostylic. Unique features in Gymnarchus not present in any known living teleost are: (1) presence of a pila antotica secundaria, (2) splitting of the common truncus hyomandi-bularis of other fishes into more or less three separately originating nerves—n. mandibularis, n. opercularis superficialis, and n. hyoideus, (3) formation of an extra nerve, the dorsalis cutaneus facialis, in the facialis system.

The Development of the Skull of Scyllium (Scyliorhinus) canícula L

ABSTRACT The development of the skull of Scyllium canicula has been studied from the first appearance of cartilage, through thirteen stages, up to the point at which the main features of the adult skull have been acquired. The parachordals are the first elements to chondrify, and evidence is presented confirming Goodrich’s observations con- cerning the visible traces of metameric segmentation of the metotic region of the parachordal. The auditory capsule chondrifies from the first in con- tinuity with the parachordal, to which it is attached by the anterior basicapsular commissure. The polar cartilages have not been found separate, but they appear as nodules of cartilage attached to the under surface of the anterior ends of the parachordals. The orbital cartilage becomes attached to the parachordal by means of the pila antotica, and to the trabecula at the base of the lamina orbitonasalis by means of the preoptic root. The hind wall of the pituitary fossa is formed in a complex manner, from a postpituitary commissure between the polar cartilages, and a pair of inwardly directed processes from the foremost ends of the parachordals forming the dorsum sellae. There is also a precarotid commissure, enclosing the carotid arteries in a foramen between itself and the postpituitary commissure. The basicranial fenestra has been demonstrated. Arguments are given for rejecting Allis’s view that the so-called basicranial fenestrae throughout the craniates are not homologous. Attention is called to the vacuity in the median wall of the auditory capsule through which the posterior canal bulges, and to the fact that this vacuity is not to be confused with the fora- men endolymphaticum. The relations of the glossopharyngeal nerve are described, and it is shown that its apparent passage through the cavity of the auditory capsule is to be ascribed to the fact that the lamina hypotica of the parachordal acts as a false floor to the auditory capsule, the true floor of which is in this region un- chondrified. The problem of the relations of the jaws to the brain-case is reviewed in the light of recent investigations, and a reasoned classification is attempted. It is noticed that chondrification is delayed in embryonic material collected from Naples, as compared with material of similar size and degree of development obtained from Plymouth.

The Early Development of the Chondrocranium of Salmo Fario

ABSTRACT The parachordal of Salmo fario was not observed to chondrify in two separate portions, anterior and posterior. No separate pole cartilages were found. The first part of the auditory capsule to arise is in the region where the hyomandibula will become attached to it. The hyomandibula, symplectic, and ceratohyal arise as separate cartilages. The hypohyals, basihyal, and copula arise as separate cartilages. The ceratobranchials,” hypobranchials, epibranchials, and pharyngobranchials (at least the anterior) arise as separate cartilages. The lateral commissure is formed from the post-palatine process, the prootic process, and cartilage representing the basi-trabecular process. The taenia marginalis (or orbital cartilage) arises separately. Rudimentary basal and otic processes are carried on the pterygo-quadrate. The olfactory nerve comes to traverse the orbit as a result of the relative forward displacement of the lamina orbito-nasalis.