Fig. 27. Dorsal view of the skull of a Labyrinthodont(Capitosaurus nasutus) × 1/9 (fromvon Zittel).1. premaxillae.12. postorbital.2. nasal.13. interparietal foramen.3. maxillae.14. squamosal.4. anterior nares.15. supratemporal.5. frontal.16. quadratojugal.6. prefrontal.17. quadrate.7. lachrymal.18. epi-otic.8. jugal.19. dermo-supra-occipital.9. orbit.20. exoccipital.10. parietal.21. foramen magnum.11. postfrontal.
Fig. 27. Dorsal view of the skull of a Labyrinthodont(Capitosaurus nasutus) × 1/9 (fromvon Zittel).1. premaxillae.12. postorbital.2. nasal.13. interparietal foramen.3. maxillae.14. squamosal.4. anterior nares.15. supratemporal.5. frontal.16. quadratojugal.6. prefrontal.17. quadrate.7. lachrymal.18. epi-otic.8. jugal.19. dermo-supra-occipital.9. orbit.20. exoccipital.10. parietal.21. foramen magnum.11. postfrontal.
Labyrinthodontia.The skull in Labyrinthodontia is remarkable for its extreme solidity, the large number of bones which are present, and the extent to which the roofing over ofthe temporal and other fossae has taken place. In many forms the surface of the bones is as in Crocodiles, strongly sculptured (fig. 27, right half) with ridges and grooves which probably lodged sensory organs. The bones forming the roof of the skull are generally very uniform in size, perhaps the most noticeable of them being the paired dermo-supra-occipitals (fig. 27, 19). Paired dermo-supra-occipitals occur also in certain Ganoids. The Labyrinthodont skull also bears resemblance to that of many fish in the development of a pair of long pointed epi-otics (fig. 27, 18), which remain permanently distinct from the surrounding bones. The parietals are small and enclose between them the interparietal foramen (fig. 27, 13). In some forms in which the head is protected with an armour of scutes, these do not roof over the interparietal foramen, and from this fact it has been inferred that the Labyrinthodonts had a functional pineal eye. Both supra- and infra-temporal fossae are partially or completely roofed over by the postorbitals and large supra-temporals (fig. 27, 15).
There is generally a ring of bones in the sclerotic coat of the eye. The pterygoids do not meet in the middle line, being separated by the parasphenoid. The palatines bear teeth, and in some genera (Archegosaurus) form long splints lying along the inner side of the maxillae and more or less surrounding the posterior nares. In others (Nyrania) they lie in the normal position near the middle line, one on each side of the parasphenoid. The vomers bear teeth and sometimes meet in the middle line; they are sometimes confluent with the parasphenoid. On the ventral surface of the cranium there are generally large palatal vacuities.
In the mandible there is often a well-marked postglenoid process, and the articular is generally completely ossified.
Fig. 28. A, ventral view of the cranium; B, lateral view of the cranium and mandible ofSiphonops annulatus(afterWiedersheim).1. anterior nares.9. squamosal.2. naso-premaxillae.10. exoccipital.3. frontal.11. dentary.4. parietal.12. angular.5. maxillae.13. basi-occipital and6. vomer.basisphenoid fused.7. orbit.14. posterior narial opening8. quadrate united with thesurrounded by the palatine.pterygoid in front.X. pneumogastric foramen.
Fig. 28. A, ventral view of the cranium; B, lateral view of the cranium and mandible ofSiphonops annulatus(afterWiedersheim).1. anterior nares.9. squamosal.2. naso-premaxillae.10. exoccipital.3. frontal.11. dentary.4. parietal.12. angular.5. maxillae.13. basi-occipital and6. vomer.basisphenoid fused.7. orbit.14. posterior narial opening8. quadrate united with thesurrounded by the palatine.pterygoid in front.X. pneumogastric foramen.
Gymnophiona.The skull bears a considerable resemblance to that of Labyrinthodonts, especially in the arrangement of the bones which bound the mouth cavity. The cranium isvery hard, and is covered by a complete bony roof formed mainly of the exoccipitals, parietals, frontals, prefrontals, nasals and premaxillae. The nasals and premaxillae are sometimes ossified continuously. There is a median unpaired ethmoid whose dorsal end appears at the surface wedged in between the frontals and parietals. The bone generally regarded as the squamosal[63]is very large, and it and the maxillae generally together surround the orbit, which, inEpicrium, has in it a ring of bones. The palatines form long tooth-bearing bones fused with the inner sides of the maxillae; they nearly surround the posterior nares.
The quadrate bears the knob, and the angular the cup for the articulation of the mandible,—a very primitive feature. The mandible is also noticeable for the enormous backward projection of the angular.
Anura.In Anura the skull is very short and wide owing to the transverse position of the suspensorium. There is often a small ossification representing the quadrate. Sometimes as inHylaandAlytesthere is a fronto-parietal fontanelle.
As compared with the skull in Urodela the chief characteristics of the skull of Anura are:—
1. the presence of a sphenethmoid,
2. the union of the frontals and parietals on each side,
3. the occasional occurrence of small supra- and basi-occipitals,
4. the backward growth of the maxillae and its connection with the suspensorium by means of the quadratojugal,
5. the dagger-like shape of the parasphenoid,
6. the occurrence of a definite tympanic cavity,
7. the frequent occurrence of a predentary or mento-meckelian ossification in the mandible.
The skull ofPipais abnormal, being greatly flattened and containing little cartilage. The fronto-parietals are fused, and there is no sphenethmoid. The quadrates are well developed and the squamosals and parasphenoid differ much from those of other Anura.
Hyoid and branchial arches.
In larval Amphibia the hyoid and four branchial arches are generally present, and in adult Ichthyoidea they are frequently almost as well represented as in the larva, and are of use in strengthening the swallowing apparatus. They are very well seen inSiredon, and consist of a hyoid attached by ligaments to the suspensorium, followed by four branchial arches of which the first and second are united by a copula (fig. 29, D, 8), while the third and fourth are not. The hyoid is not always the largest and best preserved of the arches, for sometimes as inSpelerpesone of the branchials is far larger than the hyoid. Four branchial arches occur inSirenas inSiredon, but inProteusthere are only three.
In some larval Labyrinthodontia (Branchiosaurus) four branchial arches are known to occur, and their arrangement is almost precisely similar to that inSiredon.
In Gymnophiona the remains of only three branchial arches occur in addition to the hyoid. The four arches are all very similar to one another, each consists of a curved rod of uniform diameter throughout. The hyoid is united with the first branchial arch, but has no attachment to the cranium.
In larval Anura (fig. 29, C) the arrangement of the hyoid and branchial arches is much as in Urodela. In the adult, however, the ventral parts of all the arches unite, forming a compact structure, thebasilingual plate(fig. 29, B, 1).
Fig. 29. Visceral arches of Amphibia.A.Molge cristata(afterParker).B.Rana temporariaadult (afterParker).C. Tadpole ofRana(afterMartin St Ange).D.Siredon pisciformis(afterCredner).In each case the ossified portions are slightly shaded, while the cartilaginous portions are left white.1. basilingual plate.5. third branchial arch.2. hyoid arch.6. fourth do.3. first branchial arch.7. thyro-hyal.4. second do.8. copula.
Fig. 29. Visceral arches of Amphibia.A.Molge cristata(afterParker).B.Rana temporariaadult (afterParker).C. Tadpole ofRana(afterMartin St Ange).D.Siredon pisciformis(afterCredner).
In each case the ossified portions are slightly shaded, while the cartilaginous portions are left white.1. basilingual plate.5. third branchial arch.2. hyoid arch.6. fourth do.3. first branchial arch.7. thyro-hyal.4. second do.8. copula.
The dorsal parts of the first three branchial arches disappear, but those of the fourth become ossified and form the short, stout thyro-hyals or posterior cornua. The dorsal parts of the hyoid arch in the adult form a pair of long bars, the anteriorcornua, which are united to the periotic region of the skull in front of the fenestra ovalis either by short ligaments or by fusion as inBufo. InPipaandXenopusthe first and second branchial arches persist as well as the fourth (thyro-hyal), but inPipathe hyoid is wanting.
Ribs.
Ribs are generally very poorly developed in Amphibia. In Anura they are in most cases absent; when present they generally form minute unossified appendages attached to the transverse processes, but inDiscoglossusandXenopusthe anterior vertebrae are provided with distinct ribs. In Urodela and Labyrinthodontia they are generally short structures, each as a rule attached to the vertebra by a bifurcated proximal end. The number of rib-bearing vertebrae varies, but the first and the posterior caudal vertebrae are always ribless. The anterior caudal vertebrae too are generally ribless, but sometimes a few of them bear small ribs. InSpelerpesthe last two trunk vertebrae are ribless, and hence may be regarded as lumbar vertebrae.
In Gymnophiona ribs are better developed than in any other Amphibia; they occur on all the vertebrae except the first and last few, and are attached to the transverse processes, sometimes by single, sometimes by double heads.
Sternal ribs are almost unknown in Amphibia, but traces of them occur inMenobranchus.
Sternum.
In Amphibia the sternum is not very well developed; sometimes as in Gymnophiona andProteusno traces of it occur, and in the Urodela it is never ossified. It is always very intimately related to the pectoral girdle. In the Salamandrina it has the form of a broad thin plate of cartilage, grooved and overlapped by the coracoid.
Fig. 30. Shoulder-girdle and sternum ofA. An old male common Frog (Rana temporaria).B. An adult femaleDocidophryne gigantea(afterParker).In both A and B the left suprascapula is removed. The parts leftunshaded are ossified; those marked with small dots consist of hyalinecartilage, those marked with large dots of calcified cartilage.1. calcified cartilage of7. clavicle.suprascapula.8. glenoid cavity.2. ossified portion of9. coracoid foramen.suprascapula.10. episternum.3. scapula.11. omosternum.4. coracoid.12. sternum.5. epicoracoid.13. xiphisternum.6. precoracoid.
Fig. 30. Shoulder-girdle and sternum ofA. An old male common Frog (Rana temporaria).B. An adult femaleDocidophryne gigantea(afterParker).
In both A and B the left suprascapula is removed. The parts leftunshaded are ossified; those marked with small dots consist of hyalinecartilage, those marked with large dots of calcified cartilage.1. calcified cartilage of7. clavicle.suprascapula.8. glenoid cavity.2. ossified portion of9. coracoid foramen.suprascapula.10. episternum.3. scapula.11. omosternum.4. coracoid.12. sternum.5. epicoracoid.13. xiphisternum.6. precoracoid.
In most Anura the sternum consists of a number of partsarranged in series. At the anterior end is a flat cartilaginous plate with a bony basal stalk. This plate is called the episternum, and its stalk the omosternum. The continuity of the sternum is now interrupted by a pair of cartilaginous structures, the epicoracoids, which are shoulder-girdle elements, and represent the unossified ventral ends of the coracoids. In some cases cartilaginous epiprecoracoids can also be distinguished. Further back is the long sternum proper, while last comes the xiphisternum, a broad expanded plate of cartilage.
In some Anura such asPipaandHylathe number of sternal elements is considerably reduced.
Appendicular Skeleton.
Pectoral girdle.
The most primitive Amphibian shoulder-girdle is found in the Urodela. It consists of a dorsal element, the scapula, a posterior ventral element, the coracoid, and an anterior ventral element, the precoracoid. The clavicle is not developed, and the two coracoids overlap in the middle line. The shoulder-girdle remains largely cartilaginous but the proximal end of the scapula is ossified, and the ossification may extend through part of the coracoid and precoracoid.
In Labyrinthodontia there is an exoskeletal ventral buckler formed of three plates, a median one, which probably represents an interclavicle, and two lateral ones, which are probably clavicles. Traces of endoskeletal structures, probably corresponding to the precoracoid and scapula, are also known in some cases. The Gymnophiona and some of the Labyrinthodontia have lost the pectoral girdle and limbs.
The ossification of the shoulder-girdle has gone on much further in Anura than it has in Urodela. Clavicles are present and the scapula and coracoid of each side are ossified from separate centres. The distal part of the scapula forms a large imperfectly ossified plate, the suprascapula.
The shoulder-girdle of Anura is however subject to considerable variations. In the Toads (Bufonidae) the epicoracoids or unossified ventral ends of the coracoids and precoracoids overlap in the middle line (fig. 30, B, 5). This arrangement is calledArciferous. In the Frogs,—Ranidae, and other forms belonging to the groupFirmisternia,—the epicoracoids do not overlap but form a narrow cartilaginous bar separating the ventral ends of the coracoids (fig. 30, A, 5).
Anterior limb.
In many Amphibia and especially in the Urodela the anterior limb has a very simple unmodified arrangement. The humerus is straight and of moderate length, its ends are rounded for articulation on the one hand with the shoulder-girdle, and on the other hand with the radius and ulna. In the Urodela the radius and ulna are distinct. In the Anura they have fused, though the line of junction of the two is not obliterated. Their proximal ends are hollowed for articulation with the convex end of the humerus.
The manus in all recent Amphibia agrees in never having more than four complete digits, but is subject to considerable variation, this statement applying especially to the carpus.
In the larva of Salamandra (fig. 31, A), except that the pollex is absent[64], the manus retains completely the condition which is generally regarded as primitive for the higher Vertebrata. It consists of an anterior row of three elements, the ulnare, intermedium, and radiale, and a posterior row of four, the carpalia 2, 3, 4, and 5. Interposed between the two rows is a centrale.Menobranchushas a similar very simple carpus. In most other Amphibia this simplicity is lost. This loss may be due to:—
(a) fusion of certain structures, e.g. in the adultSalamandrathe intermedium and ulnare have fused,
(b) displacement of structures, e.g. inBufo viridis, the centrale has been pushed up till it comes to articulate with the radius,
(c) the development of supernumerary elements, especially of extra centralia. InMegalobatrachustwo or even three centralia sometimes occur.
Fig. 31. A, Right Antibrachium and Manus of a larval Salamander(Salamandra maculosa) (afterGegenbaur).B, Right Tarsus and adjoining bones ofMolge sp.(afterGegenbaur).1. radius.11. tibia.2. ulna.12. fibula.3. radiale.13. tibiale.4. intermedium.14. intermedium.5. ulnare.15. fibulare.6. centrale.16. centrale.7. carpale 2.17. tarsale 1.8. " 3.18. tarsalia 4 and 5 fused.9. " 4.I. II. III. IV. V. digits.10. " 5.
Fig. 31. A, Right Antibrachium and Manus of a larval Salamander(Salamandra maculosa) (afterGegenbaur).B, Right Tarsus and adjoining bones ofMolge sp.(afterGegenbaur).1. radius.11. tibia.2. ulna.12. fibula.3. radiale.13. tibiale.4. intermedium.14. intermedium.5. ulnare.15. fibulare.6. centrale.16. centrale.7. carpale 2.17. tarsale 1.8. " 3.18. tarsalia 4 and 5 fused.9. " 4.I. II. III. IV. V. digits.10. " 5.
In the great majority of Amphibia while one digit, probably the first, is absent, the other four digits are well developed. In the forms however with degenerate limbs likeAmphiuma,SirenandProteusthe number of digits is stillfurther reduced. InSirenthere are three or four, inProteusthree, and inAmphiumatwo or three digits in the manus.
In Anura the pollex is represented only by a short metacarpal. There are sometimes traces of a prepollex. The carpus often has two centralia and the intermedium is absent.
In Labyrinthodontia the limbs are generally very simple and resemble those of Urodela. In some forms, however, the manus differs from that of all living Amphibia in possessing five well-developed digits.
Pelvic girdle.
The simplest Amphibian pelvis is that of some of the Labyrinthodontia; thus inMastodonsaurusit consists dorsally of a short broad ilium placed vertically and attached to the sacrum, and ventrally of a small pubis and of a large ischium meeting its fellow in the middle line. In some Labyrinthodonts the pubes as well as the ischia meet in a ventral symphysis, and in many there are no obturator foramina. InSiren, Gymnophiona and some Labyrinthodontia the pelvic girdle and limbs are absent.
In Urodela the ventral element of the pelvis on each side forms a flat plate which meets its fellow of the opposite side. The anterior part of the plate, representing the pubis, generally remains cartilaginous throughout life; the posterior part representing the ischium is in almost every case well ossified. Attached to the anterior end of the pubes there is an unpaired bifid cartilaginous structure, the epipubis. The ilia are vertically placed.
In most Anura the pelvis is peculiarly modified in correlation with the habits of jumping. The long bone generally called the ilium is placed horizontally and is attached at its extreme anterior end to the sacrum. The ischium is ossified and distinct. Ventrally in front of the ischium there is a tract of unossified cartilage which is often regarded as thepubis. InXenopus, however, the bone corresponding to the ilium of the Frog is seen to ossify from two centres, one forming the ilium, the other, which lies at the symphysis, being apparently the pubis. This makes it probable that the so-called ilium of the Frog is really to be regarded as an ilio-pubis, and renders the homology of the cartilaginous part uncertain, but it probably corresponds to the acetabular bone of mammals. InXenopusalso there is a minute epipubis similar to that of Urodeles.
Posterior limb.
In Urodela the posterior limb (fig. 31, B) closely resembles the anterior limb, but is even less removed from the primitive condition of the higher vertebrates in the fact that all five digits are commonly present. The tibia and fibula are short bones approximately equal in size. In some cases the number of digits is reduced. Thus inMenobranchusthe pes has four digits, inProteusit has two, and inAmphiumatwo or three, while inSirenthe posterior limbs have atrophied.
In correlation with their habits of jumping, the posterior limbs in Anura are much lengthened and considerably modified. The tibia and fibula are completely fused. The intermedium is absent, while the tibiale and fibulare are greatly elongated. Tarsalia 4 and 5 are absent. Five digits are always present, and there is a prehallux formed of two or more segments.
In general the posterior limbs in Labyrinthodontia bear the closest resemblance to the anterior limbs; in some cases three centralia are found.
In Ichthyoidea, and in most Labyrinthodontia, the cartilages of the carpus and tarsus remain unossified; in Salamandrina and in Anura they are generally ossified.
This great group includes the Reptiles and Birds and forms the second of the three into which the Gnathostomata may be divided. There is nearly always a strongly-developed epiblastic exoskeleton which has the form of scales or feathers, and in some cases a dermal exoskeleton is also well developed. In living forms the notochord never persists, being replaced by vertebrae, but in some extinct forms the centra are notochordal. The vertebral centra are ossified, and only in exceptionally rare cases have terminal epiphyses. The skull is well ossified and has membrane bones incorporated in its walls.
The occipital segment is completely ossified, and an interorbital septum or bony partition separating the two orbits is usually developed to a greater or less extent. The skull generally articulates with the vertebral column by a single occipital condyle into the composition of which the exoccipitals and basi-occipital enter in varying proportions. The pro-otic ossifies, and either remains distinct from the epi-otic[65]and opisthotic throughout life, or unites with them only after they have fused with the adjacent bones. The hyoid and branchial arches are much reduced; and the representative of the hyomandibular is connected with theauditory apparatus, forming the auditory ossicles[66]. Each ramus of the mandible always consists of a cartilage bone, the articular, and several membrane bones. The mandible articulates with the cranium by means of a quadrate. The ribs in Birds and some Reptiles bearuncinate processes, i.e. small, flat, bony or cartilaginous plates projecting backwards from their posterior borders. The sternum is not transversely segmented as in mammals, and there are commonly distinct cervical ribs. The ankle joint is intertarsal, or situated between the proximal and distal row of tarsal bones, not cruro-tarsal as in Mammalia.
Class I. Reptilia[67].
The axial skeleton is generally long, and that of the limbs frequently comparatively short, or sometimes absent.
The exoskeleton generally has the form of epidermal scales, which are often combined with underlying bony dermal plates or scutes and may sometimes form a continuous armour. Neither feathers nor true hairs are ever present. The vertebral column is generally divisible into the five usual regions. The centra of the vertebrae vary enormously, and may be amphicoelous, procoelous, opisthocoelous or flat, but they never have saddle-shaped articulating surfaces. The quadrate is always large, and is sometimes fixed, sometimes movable. A transpalatine bone uniting the pterygoid and maxillae is generally present.
Free ribs are often borne along almost the whole length of the trunk and tail, and often occur attached to the cervical vertebrae. The sacrum is generally composed of two vertebrae which are united with the ilia by means of expanded ribs. The sternum is rhomboidal, and may either be cartilaginousor formed of cartilage bone, but never of membrane bone; it differs from that of birds also in the fact that it does not ossify from two or more centres. An interclavicle is generally present. There are always more than three digits in the manus, and never less than three in the pes. In all living reptiles the ilia are prolonged further behind the acetabula than in front of them, and the bones of the pelvis remain as a rule, distinct from one another throughout life.
The pubes (pre-pubes) and ischia both commonly meet in ventral symphysis, and the acetabula are wholly or almost wholly ossified. The metatarsals are not ankylosed together.
Order 1.Theromorpha[68].
This order includes a number of mainly terrestrial, extinct reptiles, which differ much from one another, and show remarkable points of affinity on the one hand with the Labyrinthodont Amphibia, and on the other with the Mammalia. The vertebrae are nearly always amphicoelous and sometimes have notochordal centra. The skull is short and has the quadrate immovably fixed. There is an interparietal foramen, and generally large supratemporal fossae bounded by supratemporal arcades, but with no infratemporal[69]arcades;Elginiahowever has the whole of the temporal region completely roofed over.
The teeth are placed in distinct sockets and are very variable in form, the dentition sometimes resembling the heterodont dentition of mammals. The humerus has distinct condyles and an ent-epicondylar foramen[70]as in many mammals.
The pubis is fused with the ischium, and both pectoral and pelvic girdles are remarkably solid. The obturator foramenis remarkably small or even absent. The anterior ribs have two articulating surfaces, and each articulates by its tuberculum with the transverse process, and by its capitulum with the centrum as in mammals.
These reptiles occur chiefly in deposits of Triassic and Permian age. Some of the best known genera areDicynodon,Udenodon,Placodus,PariasaurusandGalesaurus. They will be noticed in the general account of the skeleton in reptiles.
Order 2.Sauropterygia.
This order includes a number of extinct marine reptiles, devoid of an exoskeleton. The tail is short, the trunk long, and the neck in the most typical forms extremely long. The vertebrae have slightly biconcave, or nearly flat centra. The skull is relatively small and has large supratemporal fossae. The teeth are placed in distinct sockets, and are generally confined to the margins of the jaws; they are sharp and curved and are coated with grooved enamel. The premaxillae are large, and there is an interparietal foramen. The quadrate is firmly united to the cranium. The anterior nares are separate and are placed somewhat close to the orbits. There is no ossified sclerotic ring. The palatines and pterygoids meet the vomers, and more or less completely close the palate, and in some forms, e.g.Plesiosaurus, there is a distinct parasphenoid. Thoracic ribs are strongly developed and each articulates with its vertebra by a single head. The cervical vertebrae have well-marked ribs, which articulate only with the centra, in this respect differing from those of Crocodiles. The caudal vertebrae bear both ribs and chevron bones, and abdominal splint-ribs are largely developed.
In the shoulder-girdle the coracoids are large and meet in a ventral symphysis; precoracoids and a sternum are apparently absent, but parts generally regarded[71]as the clavicles andinterclavicle are well developed. In the pelvis, the pubes and ischia meet in a long symphysis. The limbs are pentedactylate, and in the best known forms, the Plesiosauridae, form swimming paddles.
The Sauropterygia occur in beds of Secondary age, and some of the best known genera arePlesiosaurus,PliosaurusandNothosaurus.
Order 3.Chelonia.
In the Tortoises and Turtles the body is enclosed in a bony box, formed of the dorsal carapace, and a flat ventral buckler, the plastron. Except inDermochelysthe carapace is partly formed from the vertebral column and ribs, partly from dermal bones. Both carapace and plastron are, except inDermochelys,Trionyxand their allies, covered with an epidermal exoskeleton of horny plates, which are regularly arranged, though their outlines do not coincide with those of the underlying bones. The thoracic vertebrae have no transverse processes, and are quite immovably fixed, but the cervical and caudal vertebrae are very freely movable. There are no lumbar vertebrae. The skull is extremely solid, and frequently has a very complete false roof. Teeth have been detected in embryos ofTrionyxbut with this exception the jaws are toothless, and are encased in horny beaks. The quadrate is firmly fixed. The facial part of the skull is very short, and the alisphenoidal and orbitosphenoidal regions are unossified. In living forms there are no separate nasal bones, while large prefrontals and postfrontals are developed. There is a comparatively complete bony palate chiefly formed of the palatines and pterygoids. The anterior nares are united and placed at the anterior end of the skull, and the premaxillae are very small. There is no transpalatine bone and the vomer is unpaired. The dentaries are generally fused together.
There are ten pairs of ribs, and each rib has only a singlehead and is partially attached to two vertebrae; there are no cervical or sternal ribs. There is no true sternum.
The three anterior elements of the plastron are respectively homologous with the interclavicle and two clavicles of other reptiles, while the remaining elements of the plastron are probably homologous with the abdominal ribs of Crocodiles. The pectoral girdle lies within the ribs, and the precoracoids and coracoids do not meet in ventral symphyses. The scapula and precoracoid are ossified continuously. The pubis probably corresponds with the pre-pubis of Dinosaurs. There are four limbs each with five digits.
The order includes three suborders:—
Suborder (1).Trionychia.
The carapace and plastron have a rough granular surface covered with skin and without any horny shields.
The plastron is imperfectly ossified, and marginal bones may be absent, or if present are confined to the posterior portion of the carapace. The pelvis is not united to the plastron. The cranium has not a complete false roof and the head can be drawn back under the carapace.
The first three digits of both manus and pes bear claws, and the fourth digit in each case has more than three phalanges. The most important genus isTrionyx.
Suborder (2).Cryptodira.
The carapace and plastron vary in the extent to which they are ossified, and except inDermochelys[72]and its allies are covered by horny plates. Marginal bones are always present. The head can generally be drawn back under the carapace. The pelvis is not firmly united to the plastron. The cranium often has a complete false roof, and in the mandibular articulation the cup is borne by the cranium, and the knob by the mandible. Among the more important genera areDermochelys,Chelone, andTestudo.
Suborder (3).Pleurodira.
The carapace and plastron are strongly ossified, and firmly united to the pelvis. The head and neck can be folded laterally under the carapace, but cannot be drawn back under it. The cranium has a more or less complete false roof, and in the mandibular articulation the knob is borne by the cranium, and the cup by the mandible.Chelysis a well-known genus.
Order 4.Ichthyosauria[73].
The order includes a number of large extinct marine reptiles whose general shape is similar to that of the Cetacea. The skull is enormously large, and the neck short. The tail is very long, and is terminated by a large vertically-placed bilobed fin, the vertebral column running along the lower lobe. The very numerous vertebrae are short and deeply biconcave. The vertebral column can be divided into caudal and precaudal regions only, as the ribs which begin at the anterior part of the neck are continued to the posterior end of the trunk without being connected with any sternum or sacrum. The precaudal vertebrae bear two surfaces for the articulation of the ribs, while in the caudal vertebrae the two surfaces have coalesced. The caudal region is also distinguished by its chevron bones. The vertebrae have no transverse processes, and the neural arches are not firmly united to the centra, and have only traces of zygapophyses. The atlas and axis are similar to the other vertebrae, but there is a wedge-shaped intercentrum between the atlas and the skull, and another between the atlas and the axis. The skull is greatly elongated (fig. 32) and pointed, mainly owing to the length of the premaxillae. The orbits are enormous, and there is a ring of bones in the sclerotic (fig. 32, 15). The anterior nares are very small; andare placed far back just in front of the orbits. There is an interparietal foramen, and the supratemporal fossae (fig. 32, 9) are very large, while there are no infratemporal fossae. An epipterygoid occurs. The quadrate is firmly fixed to the cranium, and there is a large parasphenoid. There are large prefrontals, but the frontals are very small. The very numerous teeth are large and conical, and are placed in continuous grooves without being ankylosed to the bone. They are confined to the jaw-bones.
Fig. 32. Lateral (below) and dorsal (above) views of the skull of anIchthyosaurus. (Modified from Deslongchamps.)1. premaxillae.12. squamosal.2. maxillae.13. supratemporal.3. nasal.14. quadratojugal.4. prefrontal[74].15. sclerotic ring.5. frontal.16. postorbital.6. postfrontal[74].17. jugal.7. anterior nares.18. lachrymal.8. orbit.19. dentary.9. supratemporal fossa.20. articular.10. interparietal foramen.21. angular.11. parietal.
Fig. 32. Lateral (below) and dorsal (above) views of the skull of anIchthyosaurus. (Modified from Deslongchamps.)1. premaxillae.12. squamosal.2. maxillae.13. supratemporal.3. nasal.14. quadratojugal.4. prefrontal[74].15. sclerotic ring.5. frontal.16. postorbital.6. postfrontal[74].17. jugal.7. anterior nares.18. lachrymal.8. orbit.19. dentary.9. supratemporal fossa.20. articular.10. interparietal foramen.21. angular.11. parietal.
The ribs are long, and the anterior ones have capitula andtubercula. There is no sternum, but the ventral body wall is strengthened by a complex system of abdominal splint ribs.
The pectoral girdle is strongly developed, the scapulae are narrow, the coracoids broad, and meet ventrally without overlapping. There are probably no precoracoids, but clavicles and a T-shaped interclavicle are well developed.
The limbs are very short, and completely modified into swimming paddles. The humerus and femur are both short, while the radius and ulna, tibia and fibula are generally still further reduced to the form of short polygonal bones.
The digits are formed of longitudinal series of very numerous small bones. The number of digits is five, but there sometimes appear to be more owing to the bifurcation of certain of them, or to the addition of marginal bones, either to the radial or ulnar side of the limb. The humerus has no foramen, and both humerus and femur are unique in that they are distally terminated by concave surfaces instead of by convex condyles. The pelvic limb is much smaller than the pectoral. The pelvis has no bony connection with the vertebral column, and all the component bones are small and rod-like.
The Ichthyosauria are confined to beds of Secondary age and by far the best known genus isIchthyosaurus.
Order 5.Rhynchocephalia.
This order includes the livingSphenodon(Hatteria) and various extinct forms. The general shape of these animals is lizard-like and the tail is long.
The vertebrae are amphicoelous or sometimes nearly flat, and the notochord sometimes persists to some extent.Proterosaurusdiffers from the other members of the order in having opisthocoelous cervical vertebrae.
The sacrum is composed of two vertebrae. Ossified inter centra (interdorsalia) generally occur in the cervical and caudal regions, and sometimes throughout the whole vertebral column. In the skull the quadrate is immovably fixed and united to the pterygoid. The palate is well ossified, while the premaxillae which are often beak-like are never ankylosed together. The jaws may be toothless or may be provided with teeth which are usually acrodont (see p. 199). The palatines frequently bear teeth, and inProterosaurusteeth occur also on the pterygoids and vomers. The rami of the mandible are united by ligament at the symphysis except in the Rhynchosauridae, in which the union is bony. Superior and inferior temporal arcades occur.
The ribs have capitula and tubercula, and often uncinate processes (see p. 190) as in birds. A pectoral girdle and sternum, with clavicles and a T-shaped interclavicle are developed, and abdominal ribs are always found. The precoracoid is however absent. The limbs are pentedactylate.
Sphenodon[75](Hatteria) now living in some of the islands of the New Zealand group, is certainly the most generalised of all living reptiles. Though lizard-like in form it differs from all living lizards in the possession of two temporal arcades, abdominal ribs and a fixed quadrate; and is often considered to be nearly allied in many respects to the type of reptile from which all the others took their origin.
Among the better known extinct forms areProterosaurusof Permian andHyperodapedonof Triassic age.
Order 6.Squamata.
This order includes the extinct Mosasaurians, and the lizards and snakes which form the vast majority of living reptiles. The trunk may be moderately elongated and providedwith four short limbs as in lizards, or it may be limbless, extremely elongated, and passing imperceptibly into the tail. The surface is generally completely covered with overlapping horny epidermal scales, below which bony dermal scutes may be developed.
The vertebrae are procoelous, rarely amphicoelous. There are no inter centra, and the neural arches are firmly united to the centra. Additional articulating surfaces, the zygosphenes and zygantra, are often developed[76]. The sacrum is formed of two or rarely three vertebrae, or may be wanting as in Ophidia. In the skull an infratemporal arcade forming the lower boundary of the infratemporal fossa is absent, and the quadrate, except in the Chamaeleons, is movably articulated to the squamosal. The palatal vacuities are large and the nares are separate. There is often a distinct parasphenoid. The teeth are eitheracrodont(i.e. ankylosed to the summit of the jaw), orpleurodont, i.e. ankylosed to the inner side of the jaw. The thoracic ribs each have a single head which articulates with the centrum of the vertebra; while uncinate processes and abdominal ribs never occur.
A pectoral girdle and sternum may be present, or may be completely absent as in snakes. Except in snakes there are generally four pentedactylate limbs which may either form paddles or be adapted for walking.
Suborder (1).Lacertilia[77].
The body is elongated, and as a rule four short pentedactylate limbs are present, but sometimes limbs are vestigial orabsent. The exoskeleton generally has the form of horny plates, spines, or scales; while sometimes as in the Chamaeleons and Amphisbaenians it is absent. In other forms such asTiliquaandScincus, the body has a complete armour of bony scutes, whose shape corresponds with that of the overlying horny scales.
The vertebrae are procoelous, rarely as in the Geckos amphicoelous; they are usually without zygosphenes and zygantra, but these structures occur in the Iguanidae. The sacral vertebrae of living forms are not ankylosed together, and the caudal vertebrae usually have well-developed chevron bones.
In the skull[78]the orbits are separated from one another, only by an imperfectly developed interorbital septum, the cranial cavity not extending forwards between them, while the alisphenoidal region is unossified. The premaxillae may be paired or united (Amphisbaenidae), and there is usually an interparietal foramen. There may be a complete supratemporal[79]arcade bounding the lower margin of the supratemporal fossa, or the supratemporal fossa may be open below. The quadratojugal is not ossified, and the quadrate articulates with the exoccipital. There is no infratemporal arcade. There is commonly a rod-like epipterygoid[80](fig. 33, 14) connecting the pterygoid and parietal.
Teeth are always present, and may be confined to the jaws or may be developed also on the pterygoids and rarely on the palatines; they are either acrodont or pleurodont. The rami of the mandible are suturally united.
A pectoral girdle is always present, and generally also a sternum. Clavicles and a T-shaped interclavicle are commonly present, but are absent in the Chamaeleons.