Subarcualis Rectus Research Articles

The missing piece of the puzzle: larval morphology of Xenohyla truncata (Anura: Hylidae: Dendropsophini) and its implication to the evolution of Dendropsophini tadpoles

Dendropsophini is a highly diverse clade with a controversial phylogenetic and taxonomic history. Different generic arrangements have been proposed and the monophyly of several clades supported or rejected. Previous evidence suggested that larval morphology could play an important role in our understanding of the evolution and diversification of Dendropsophini, although data are missing for most lineages, including the sister group of Dendropsophus, Xenohyla. Herein we describe the internal morphology of the tadpoles of X. truncata and compare our results with available information for members of Dendropsophini and closely related lineages. We propose that the presence of a fan-like papilla in the buccopharyngeal cavity, a single element suprarostral, and a triangular process at the base of the muscular process are synapomorphies for Dendropsophini; moreover, the presence of a divided m. subarcualis rectus II–IV seems to be a synapomorphy for Pseudini and, the nasal sac insertion of the m. levator lateralis could be a synapomorphy of Dendropsophini + Pseudini.

A decharming metamorphosis: The larval and adult morphology of the common spadefoot toad, Pelobates fuscus

Anurans are a highly diverse amphibian order with a biphasic lifestyle. The larva, better known as tadpole, and the adult frog of one species typically occupy different habitats and exploit different resources. Therefore, both are subject of largely independent selection and are able to evolve stage-specific adaptations. Among others, the musculoskeletal features of the larval and adult head of anurans and the description of their specific traits are important to gain a deeper understanding of the evolution and the phylogeny of this highly diverse group. Here we present an overview of the larval and adult cranial morphology of the common spadefoot toad Pelobates fuscus. We provide a detailed description of the cranial musculoskeletal structures based on micro-computed tomography and following 3D reconstruction of two long-term stored specimens of P. fuscus and refer to striking adaptations of this species. P. fuscus inhabits a wide spectrum of habitats including mixed forests, steppes, fields and sandy areas with soft soils in close proximity to permanent water bodies. Adults have a semi-fossorial lifestyle and they are able to dig themselves in using their hindlimbs whereas the tadpoles are the largest of all European anurans. Derived larval features are the presence of an adrostral cartilage within the lower jaw, the presence of three portions of the M. subarcualis rectus I, the presence of the M. diaphragmatopraecordialis and a bipartite suprarostral cartilage. The presence of an enlarged prehallux and the co-ossification of the frontoparietal, as well as a pterygoid that borders the orbit ventrally and rostrally are striking adult features and adaptation to the semi-fossorial lifestyle. With the described morphological features, we want to underline their importance for several adaptations which may have facilitated and stimulated the evolution of anurans and provide baseline data for further investigations.

Larval Anatomy of Andean Toads of the Rhinella spinulosa Group (Anura: Bufonidae)

The Rhinella spinulosa group is a clade of toads that inhabit the Andes mountains from northern Ecuador to Patagonia. Its taxonomy was recently revised, and in its new arrangement comprises nine species, including Rhinella gallardoi, traditionally placed in a different intrageneric group. In this work we studied the larval external and internal morphology in this group, by describing for the first time tadpoles of R. achalensis, R. gallardoi, and R. vellardi, and then summarizing morphological data for R. altiperuviana, R. limensis, R. papillosa, R. spinulosa, and R. trifolium. Although we found no diagnostic larval features for the whole clade, two distinct morphs were identified. Most tadpoles were highly pigmented and slender, and their oral discs showed a long gap in the second labial tooth row; conversely, tadpoles of R. limensis and R. vellardi shared a globose body and a very short gap. Buccal and musculoskeletal features were highly conserved within the group and regarding other Rhinella, and included four lingual papillae, nonkeratinized spurs, tripartite suprarostral cartilages, quadrato-orbital commissure, and in musculature, m. subarcualis rectus II–IV invading the branchial septum III and laryngeal muscles reduced or absent.

Convergently evolved muscle architecture enables high-performance ballistic movement in salamanders.

Elastically powered ballistic movements, such as tongue projection, are common in nature, likely due to benefits such as increased acceleration and distance of movement, and decreased thermal sensitivity imparted by elastic mechanisms. Within Plethodontidae, both muscle-powered and elastically powered ballistic tongue projection occur. Thus, we examine how elastically powered ballistic tongue projection morphology has evolved from muscle powered projection at the level of the projector muscles (m. subarcualis rectus [SAR]). We find that two main SAR morphologies have evolved within Plethodontidae. The first SAR morphology is conducive to elastically powered ballistic projection. This ballistic SAR morphology has evolved multiple, independent times within Plethodontidae, and results from the correlated evolution of several traits including increased collagen aponeuroses, larger SAR muscles, and the loss of inner myofibers attaching directly to the tongue skeleton. While the independent evolution of ballistic SAR morphology has arrived at a similar anatomical design, other tongue structures such as tongue attachment and skeleton folding type varies among species with a ballistic SAR morphology. The second morphology is conducive to muscle-powered projection and is similar to morphology found in an outgroup, Salamandridae. The SAR of these species have inner myofibers that attach to the tongue skeleton, limiting projection distance, coupled with reduced collagen aponeuroses present in relatively small projector muscles. This SAR morphology has likely been retained from ancestors or may be related to feeding ecology. Overall, a ballistic SAR morphology has evolved repeatedly and independently due to the correlated evolution of several SAR traits, including the loss of inner myofibers, which is likely a defining feature of ballistic projection.

Tadpole morphology of Leptodactylus plaumanni (Anura: Leptodactylidae), with comments on the phylogenetic significance of larval characters in Leptodactylus

In this paper I summarize the morphology of the Leptodactylus plaumanni tadpoles, describing the external morphology, buccal cavity, and cranial skeleton and associated muscles. A distinctive combination of traits include the truncated snout in lateral view, dorsal fin originated anterior to the body-tail junction, submarginal papillae present in some specimens, two slight indentations in the lower lip, ceratobranchial III free from the hypobranchial plate, small ventrolateral projections of the corpus of the suprarostral cartilage, m. subarcualis rectus I with three heads, and m. subarcualis rectus II-IV inserting in ceratobranchial I and connective tissue between branchial processes II and III. The buccal cavity shares the typical features in species of the group, namely four lingual papillae, two pairs of infralabial papillae, two postnarial papillae, and one pair of not branched lateral ridge papillae. Attending to the increasing role of larval characters in phylogenetic analyses, further research is needed to understand the evolution of tadpole morphology in this genus.

The hyal and ventral branchial muscles in caecilian and salamander larvae: Homologies and evolution

Amphibians (Lissamphibia) are characterized by a bi-phasic life-cycle that comprises an aquatic larval stage and metamorphosis to the adult. The ancestral aquatic feeding behavior of amphibian larvae is suction feeding. The negative pressure that is needed for ingestion of prey is created by depression of the hyobranchial apparatus as a result of hyobranchial muscle action. Understanding the homologies of hyobranchial muscles in amphibian larvae is a crucial step in understanding the evolution of this important character complex. However, the literature mostly focuses on the adult musculature and terms used for hyal and ventral branchial muscles in different amphibians often do not reflect homologies across lissamphibian orders. Here we describe the hyal and ventral branchial musculature in larvae of caecilians (Gymnophiona) and salamanders (Caudata), including juveniles of two permanently aquatic salamander species. Based on previous alternative terminology schemes, we propose a terminology for the hyal and ventral branchial muscles that reflects the homologies of muscles and that is suited for studies on hyobranchial muscle evolution in amphibians. We present a discussion of the hyal and ventral branchial muscles in larvae of the most recent common ancestor of amphibians (i.e. the ground plan of Lissamphibia). Based on our terminology, the hyal and ventral branchial musculature of caecilians and salamanders comprises the following muscles: m. depressor mandibulae, m. depressor mandibulae posterior, m. hyomandibularis, m. branchiohyoideus externus, m. interhyoideus, m. interhyoideus posterior, m. subarcualis rectus I, m. subarcualis obliquus II, m. subarcualis obliquus III, m. subarcualis rectus II-IV, and m. transversus ventralis IV. Except for the m. branchiohyoideus externus, all muscles considered herein can be assigned to the ground plan of the Lissamphibia with certainty. The m. branchiohyoideus externus is either apomorphic for the Batrachia (frogs + salamanders) or salamander larvae depending on whether or not a homologous muscle is present in frog tadpoles.

Living in fast‐flowing water: Morphology of the gastromyzophorous tadpole of the bufonid Rhinella quechua (R. veraguensis group)

We describe the bufonid gastromyzophorous tadpoles of Rhinella quechua from montane forest streams in Bolivia. Specimens were cleared and stained, and the external morphology, buccopharyngeal structures, and the musculoskeletal system were studied. These tadpoles show a combination of some traits common in Rhinella larvae (e.g., emarginate oral disc with large ventral gap in the marginal papillae, labial tooth row formula 2/3, prenarial ridge, two infralabial papillae, quadratoorbital commissure present, larval otic process absent, mm. mandibulolabialis superior, interhyoideus posterior, and diaphragmatopraecordialis absent, m. subarcualis rectus I composed of three slips), some traits apparently exclusive for the described species of the R. veraguensis group (e.g., second anterior labial tooth row complete, lingual papillae absent, adrostral cartilages present), and some traits that are shared with other gastromyzophorous tadpoles (e.g., enlarged oral disc, short and wide articular process of the palatoquadrate, several muscles inserting on the abdominal sucker). In the context of the substantial taxonomic and nomenclatural changes that the former genus Bufo has undergone, and despite the conspicuous morphological differences related to the presence of an abdominal sucker, the larval morphology of R. quechua supports including it in the genus Rhinella and placing it close to species of the R. veraguensis assemblage.

Morphological characterization of Leptodactylus elenae tadpoles (Anura: Leptodactylidae: L. fuscus group), from central Paraguay

In this paper we characterize the morphology of Leptodactylus elenae tadpoles (L. fuscus group) from central Paraguay, considering external morphology, oral disc, buccopharyngeal cavity, skeleton and musculature (N = 17; Gosner Stage 36–37; Central, Paraguay). Specimens were fixed with 10% formaline and were prepared according to a classic staining protocol. These larvae show external traits similar to those of other taxa within the group, such as an ovoid and depressed body, left spiracle, ventromedial vent tube, and marginal papillae interrupted in a wide rostral gap. Distinctive features are the rounded snout, the distally wide and rounded tail, the labial tooth formula 2(2)/3(1), lateral constrictions absent, and the large internal marginal papillae, intercalated every two or four small external marginal papillae. Inside the buccopharyngeal cavity, traits shared with related species are the prenarial ridge, the transversal nares, two pairs of infralabial papillae, the medial ones joined at their bases, four lingual papillae, and dorsal and ventral vela with secretory epithelium. Skeletal configuration is constant within the genus, with common character states such as the quadripartite suprarostral cartilage, quadratoorbital commissure and larval crista parotica present, larval otic process absent, ceratobranchials fused to the hypobranchial plates, anterior branchial process, and basihyal absent. Leptodactylus elenae and other L. fuscus group species resemble taxa from the L. pentadactylus group, due to the configuration of the branchial process, the articular process of the palatoquadrate, the suspensorium, and the trabecular horns; this can be observed in the chondrocranium and hyobranchial skeleton morphospaces for 22 species of the genus. Finally, muscular features are also maintained at intra-genus level, with common traits such as m. mandibulolabialis superior absent, mm. diaphragmatopraecordialis, interhyoideus posterior and hyoangularis medialis present, m. subarcualis rectus I with 3 slips, and m. subarcualis rectus II–IV inserted on ceratobranchial III. Further analyses on intraspecific variation and descriptions of related larvae are needed to improve the knowledge about this diverse genus.

Experimental morphology of the feeding mechanism in salamanders.

The subarcualis rectus I muscle (SAR) in the feeding mechanism of four tiger salamanders (Ambystoma tigrinum) was removed early in ontogeny and these individuals were allowed to complete metamorphosis. This procedure resulted in postmetamorphic tiger salamanders which differed from control individuals in the size (and thus force generating capacity) of the SAR muscle. The experimental manipulation of muscle ontogeny allowed a test of previous hypotheses of SAR function in postmetamorphic individuals. Multivariate analysis of variance for kinematic variables measured from high-speed video records of feeding revealed that experimentally modified tiger salamanders did not protract the hyobranchial apparatus or project the tongue from the mouth during feeding. Removal of the SAR muscle resulted in significantly reduced hyobranchial elevation in the buccal cavity and reduced maximum tongue projection distance.

The strike of the tiger salamander: quantitative electromyography and muscle function during prey capture

1. We provide a quantitative description of the motor pattern of 11 cranial muscles that control terrestrial prey capture behavior via tongue projection in the tiger salamander (Ambystoma tigrinum) and test the hypothesis that the cranial motor pattern does not differ between successful and unsuccessful strikes. 2. Simultaneous high-speed video images and electromyograms from cranial muscles were recorded during prey capture by 4 individuals. Neither the time to prey contact by the tongue nor gape cycle duration during the strike were significantly different between successful and unsuccessful strikes. In addition, 76 out of 77 parameters measured from 11 cranial muscle electromyograms during the strike showed no significant differences between successful and unsuccessful captures. We conclude that the strike of the tiger salamander shows little variability under these experimental conditions. 3. During the strike all head muscles have nearly synchronous onsets of electromyographical activity. The subarcualis rectus 1 muscle exhibits two bursts of activity: one during tongue projection and a second during tongue retraction. The genioglossus and interhyoideus muscles both show sharp single peaks of activity during the tongue projection phase. The depressor mandibulae muscle typically shows a second peak in activity coincident with the plateau phase of the gape cycle.

Metamorphosis of the feeding mechanism in tiger salamanders (Ambystoma tigrinum): the ontogeny of cranial muscle mass

Most previous research on metamorphosis of the musculoskeletal system in vertebrates has focused on the transformation of the skeleton. In this paper we focus on the transformation of the muscles of the head during metamorphosis in tiger salamanders (Ambystoma tigrinum) in order (1) to provide new data on changes in myology during ontogeny, and (2) to aid in interpreting previous data on the metamorphosis of function in the head of salamanders.The physiological cross‐sectional area of nine head muscles was calculated by measuring fibre angles, fibre lengths, and muscle mass in two samples of tiger salamanders obtained just before and just after metamorphosis. The major mouth‐opening muscles (rectus cervicis and depressor mandibulae) exhibit a significant decrease in estimated maximum tetanic tension (MTT) across metamorphosis of about 36%. The jaw‐closing muscles (adductor mandibulae internus and externus) and the head‐lifting muscles (epaxials) also decrease in MTT but not significantly. The muscles associated with tongue projection during feeding on land (the subarcualis rectus I, geniohyoideus, interhyoideus and intermandibularis) all show a slight increase in MTT at metamorphosis.Metamorphic transformation of feeding behaviour in Ambystoma tigrinum involves changes in performance, the design of skeletal elements, changes in muscle force‐generating capability, and changes in hydrodynamic design from unidirectional flow in larvae to bidirectional flow during aquatic feeding after metamorphosis. Although muscle activity patterns during aquatic feeding do not change across metamorphosis, tongue‐based terrestrial feeding involves a suite of novel muscle activity patterns, morphological characters acquired at metamorphosis, and a metamorphic increase in the masses of muscles important in tongue projection.

Metamorphosis of cranial design in tiger salamanders (Ambystoma tigrinum): A morphometric analysis of ontogenetic change.

While ontogenetic analyses of skull development have contributed to our understanding of phylogenetic patterns in vertebrates, there are few studies of taxa that undergo a relatively discrete and rapid change in morphology during development (metamorphosis). Morphological changes occurring in the head at metamorphosis in tiger salamanders (Ambystoma tigrinum) were quantified by a morphometric analysis of cranial osteology and myology to document patterns of change during metamorphosis. We employed a cross-sectional analysis using a sample of larvae just prior to metamorphosis and a sample of transformed individuals just after metamorphosis, as well as larvae undergoing metamorphosis. There were no differences in external size of the head among the larval and transformed samples. The hyobranchial apparatus showed many dramatic changes at metamorphosis, including shortening of ceratobranchial 1 and the basibranchial. The subarcualis rectus muscle increased greatly in length at metamorphosis, as did hypobranchial length and internasal distance. A truss analysis of dorsal skull shape showed that at metamorphosis the snout becomes wider, the maxillary and squamosal triangles rotate posteromedially, and the neurocranium shortens (while maintaining its width), resulting in an overall decrease in skull length at metamorphosis. These morphometric differences are interpreted in light of recent data on the functional morphology of feeding in salamanders. Morphological reorganization of the hyobranchial apparatus and shape changes in the skull are related to the acquisition of a novel terrestrial feeding mode (tongue projection) at metamorphosis. Metamorphic changes (both internal and external) that can be used to judge metamorphic condition are discussed.

Kinetics of tongue projection in Ambystoma tigrinum: Quantitative kinematics, muscle function, and evolutionary hypotheses.

The projectile tongue of caudate amphibians has been studied from many perspectives, yet a quantitative kinetic model of tongue function has not yet been presented for generalized (nonplethodontid) terrestrial salamanders. The purposes of this paper are to describe quantitatively the kinnematics of the feeding mechanism and to present a kinetic model for the function of the tongue in the ambystomatid salamander Ambystoma tigrinum. Six kinematic variables were quantified from high-speed films of adult A. tigrinum feeding on land and in the water. Tongue protrusion reaches its maximum during peak gape, while peak tongue height is reached earlier, 15 ms after the mouth starts to open. Tongue kinematics change considerably during feeding in the water, and the tongue is not protruded past the plane of the gape. Electrical stimulation of the major tongue muscles showed that tongue projection in A. tigrinum is the combined result of activity in four muscles: the geniohyoideus, Subarcualis rectus 1, intermandibularis posterior, and interhyoideus. Stimulation of the Subarcualis rectus 1 alone does not cause tongue projection. The kinetic model produced from the kinematic and stimulation data involves both a dorsal vector (the resultant of the Subarcualis rectus 1, intermandibularis posterior, and interhyoideus) and a ventral vector (the geniohyoideus muscle), which sum to produce a resultant anterior vector that directs tongue motion out of the mouth and toward the prey. This model generates numerous testable predictions about tongue function and provides a mechanistic basis for the hypothesis that tongue projection in salamanders evolved from primitive intraoral manipulative action of the hyobranchial apparatus.

Tongue evolution in the lungless salamanders, family plethodontidae I. Introduction, theory and a general model of dynamics

Plethodontid salamanders capture prey by projecting the tongue from the mouth. An analysis of theoretical mechanics of the hyobranchial skeleton is used to formulate a working hypothesis of tongue movements. Predictions that the skeletal elements of the tongue are included in the projectile and that the hyobranchial skeleton is folded during projection are central to the analysis. When decapitated in a particular way, salamanders project the tongue, and it is not retracted. When these heads are fixed and sectioned, examination confirms the predications. In turn, these observations are used to refine the working hypothesis and to generate a general model of tongue dynamics for plethodontids. Muscles performing the major roles of projection (subarcualis rectus I) and retraction (rectus cervicis profundus) are identified. The skeleton is folded passively along a morphological track having the form of a tractrix. Predictions concerning the shape of the track and the exact configuration of the folded skeleton are confirmed by study of sectioned material. The skeleton unfolds along the track during retraction and is spread into the resting state. The model developed herein will be used as a basis for predictions concerning selection patterns in the family and for analytical purposes in comparative and evolutionary studies.

The feeding system of terrestrial tiger salamanders (Ambystoma tigrinum melanostictum baird).

High speed cinematography was used to record the feeding activities of terrestrial Ambystoma tigrinum melanostictum. A description of these activities based on films of more than 50 feeding sequences is presented, and the mechanical units involved are defined, described, and functionally analyzed. Evolutionary implications of the feeding system are discussed. In a typical feeding sequence, A. t. melanostictum stations and maintains its lower jaw 3-5 mm from the prey. The mouth is then opened to form a gape of ∼60° by raising the anterior end of the flexed skull and by elevating and advancing the trunk while the mental symphysis of the lower jaw remains stationary. As the mouth opens the bulging tongue is recontoured so that the posterior glandular region becomes the tip of the fully protruded tongue, which may extend 3 to 7 mm beyond the symphysis. Dorsally the protruded tongue has a deep central depression and pronounced anterolateral rims. The anterior rim collapses on contact, thereby engulfing the prey in a sticky trough that retains it during tongue withdrawal. The cervical region is then flexed and the skull snaps downward. If the prey resists the tongue and is captured by marginal teeth, A. t. melanostictum relies on repeated tongue protraction and retraction, in some cases accompanied by inertial feeding. Swallowing involves gular expansion and contraction, and is accompanied by eye depression. When the mouth is opened during ingestive activities, the lower jaw remains in place. Apparently, A. t. melanostictum uses the dorsal trunk, the cucullaris major and the robust heads of the depressor mandibulae muscles to open the mouth. During skull elevation the lower jaw is partially immobilized by the geniohyoideus, and rectus cervicis superficialis muscles. The subarcualis rectus I muscles are prime movers in tongue projection. Hebosteoypsiloideus muscles assist in tongue protrusion by slackening the rectus cervicis profundus muscles that would otherwise restrict anterior displacement of the otoglossal cartilage and copula. Tongue contouring is performed by the complex genioglossus musculature. Sublingual and anterolingual sinuses facilitate protrusion and contouring by providing space and lubrication. Rectus cervicis muscles (profundus and superficialis) are responsible for tongue withdrawal. Closure of the mouth is accomplished by the four levator mandibulae muscles, and again the lower jaw is immobilized, mostly by ventral longitudinal muscles. Skull-trunk elevation during prey capture and ingestion was also observed and filmed in several other species of Ambystoma, in Dicamptodon ensatus, and in two salamandrid species. Apparently raising and straightening the craniovertebral axis, while the mental symphysis retains contact with the substratum, is a common feature of urodele feeding systems, and does not require peculiar morphological adaptations.

On the Head of the Liopelmid Frog, Ascaphus Truei

This paper gives the first account of the larval cranial anatomy of either of the genera of Liopelmid frogs. A single, partly grown larva of Ascaphus truei, Stejneger, has been studied in transverse sections and in two-dimensional reconstructions. Its chondrocranium, jaws, gill arches, and head muscles are described and figured. Comparisons are made throughout with similar structures of Urodeles and certain other frogs, particularly Discoglossus pictus and Kana temporaria. A summary of the characters which Ascaphus shares with the Urodeles is given on pp. 175-7 and with Discoglossus on pp. 177-8. The reader is referred to these lists as an important part of this summary. Noble (1931, &c.) considers Ascaphus (with Liopelma) to be one of the two most primitive living frogs. The findings of this paper are in full agreement with this view. Thus larval Ascaphus is shown to be a persistently primitive ‘link-animal’ whose cranial structures, in almost every case, differ from those of other frogs--often radically--and throw much light on the evolution of the modern-type frog tadpole from the unknown (larval) ancestor. Ascaphus is shown to have more characters in common with the Urodeles than any other frog larva yet described. Most of these are probably a simple retention of an ancestral Amphibian plan which led on to the frogs and Urodeles (contrast the writings of Holmgren and Säve-Söderbergh). Others seem to link these two orders even more closely together. Such are: (1) The presence of ‘urobranchial’ prongs on the basibranchial copula and the attachment to them of Subarcuales obliqui and Eecti cervicis muscles; (2) the presence of a pair of Branchio-hyoideus externus muscles and other similarities of the musculature. The relationship of Ascaphus to the Gymnophiona is far less marked. Ascaphus, however, has remained more primitive than the present-day Urodeles by retaining: (1) a? Vth gill bar, with its Subarcualis rectus and S. obliquus muscles, and (2) four pairs of S. obliqui muscles instead of two. In these points it is, in fact, the most primitive living tetrapod. Ascaphus is, however, somewhat specialized in relation to a sucker mechanism and to a peculiar method of larval progression which it employs. These have led to an exaggerated autostyly of the palatoquadrate which has developed an additional fusion to the anterior tip of the auditory capsule; to a rigid fusion of the central part of the supra-rostral system to the skull; to the general heavy build of the head cartilages and to the great size of several of the mandibular and hyoid muscles; to a general consolidation and widening of parts of the hyobranchial apparatus; to a widening of the posterior jaw cartilages and the development from them of unique posterior spurs. A pre-oral mouth cavity and a long ‘posterior narial tube’ to the inner nostril are also parts of this specialization. Among the frogs Ascaphus is shown to be most nearly related to the Discoglossidae, which appear to have been derived from an ancestor with many Ascaphus-like characters. This is in further agreement with Noble's classification and is particularly true of Discoglossus pictus. Ascaphus is unique among frogs in the posterior position of its splanchnic head structures; see the list on p. 147. A forecast is made of the probable evolution of the moderntype tadpole's jaw system from that of the unknown ancestor and this is diagrammatically summed up in Text-fig. 7, pp. 158-9. Evidence is collected to show that the ‘anterior basal process’ (= commissura quadrato-cranialis anterior) is not an ethmoidal structure by origin, as has been held up to now, and consequently Säve-Söderbergh's use of it to explain an ethmoidal structure in a Stegocephalian Amphibian is criticized. An account of the muscles has been given in summary form on pp. 126 to 146 and cannot be further condensed here. But it may be noted that Edgeworth's theories (1935) of the primitive muscular content of a single branchial segment break down when applied to Ascaphus. It is now probable that a single segment could simultaneously contain a Subarcualis rectus, a S. obliquus, and a Transversus ventralis muscle. Further, Edgeworth's term ‘Transversus ventralis II’ must be changed to ‘S. obliquus II’ in the frogs and his ‘S. rectus IV’ must probably be changed to ‘S. recti IV, III, and II’ in the Urodeles.