Sub-Class X.–PYTHONOMORPHA.

The skull possesses many of the essential features of the typical lizards. The premaxillaries, frontals, and parietals are fused into unpaired bones. There is an interparietal foramen. The nostrils are dorsal, bordered by the premaxillae, nasals, prefrontals, and maxillaries. The quadrato-jugal arch is incomplete, and the orbit is posteriorly confluent with the infratemporal fossa, but a supratemporal space is shut off by the single arch, which is composed of the postfrontal, squamosal, and supratemporal. The latter is interposed between, and connects the squamosal and quadrate with the latero-posterior branch of the parietal. There is a space between this parieto-squamosal arcade and the epi-otic, which is fused with the lateral wing of the lateral occipital bone. The foramen magnum is bordered by the two supra-occipital, lateral occipital, and the unpaired basioccipital bones; the condyle is triple. The quadrate is movable, articulating with the squamosal and laterally expanded epi-otic. There is no bony connexion of the quadrate with the jugal, which is restricted to its anterior half, and attached to the maxillary and lacrymal. The quadrato-jugal is absent as a separate bone; it is probably fused with the anterior surface of the quadrate, as indicated by a perforation of the quadrate, resembling in this respect the Rhynchocephalia. The vomers are long, and separate the elongated choanae from each other. The palatines separate the vomers from the pterygoids, which enclose a long median vacuity and are not connected with the quadrates. The teeth are conical, and stand near the inner margin of the jaws upon little prominences, with which they fuse. Some genera have teeth upon the pterygoids also.

The vertebrae are very numerous and are mostly procoelous. They are noteworthy for the possession of an additional anterior and a posterior pair of articulating processes on the neural arches, homologous with the zygosphenes and zygantra of Snakes and Iguanidae (see p. 582). Intercentra are absent, except in the tail. The ribs have no tubercula, and articulate with the centra of the vertebrae to which they belong.

The pectoral arch is strong. The scapulae are short and broad; the coracoids, fused with the precoracoids, except for a notch, are flat and broad, and meet ventrally; posteriorly they articulate upon the anterior margin of the flat sternum, to the lateral margin of which are attached several ribs. Clavicles and interclavicle seem to be absent. Abdominal ribs are likewise absent. The pelvic girdle is feeble; the ilia, ischia, and pubes are loosely connected with each other, the pairs of ventral elements meeting also in the middle line. The ilia are loosely attached to two vertebrae in the Dolichosauri; in the Mosasauri they have lost this connexion. Both anterior and posterior limbs are transformed into pentadactyle paddles, with much shortened and broadened bones of the arms and legs. The digits are to a certain extent hyperphalangeal, since several of them possess five phalanges.

The Pythonomorpha are undoubtedly allied to the Sauria, but they are certainly not their ancestors, since typical Autosauri occur in the Lower Chalk; nor are the Snakes their descendants, in spite of many convergent resemblances. We consider them to be the marine collateral branch of the Sauria, which rapidly developed highly specialised, often very large forms, restricted to the Cretaceous epoch, with a wide, cosmopolitan distribution.

Order I. DOLICHOSAURI.

This older group is characterised by the sutural symphysial connexion of the two mandibles and by the possession of two sacral vertebrae. The body is snake-like. Pleurodont. Dolichosaurus longicollis of the Lower Chalk of Kent and Sussex; total length about 3 feet, with about seventeen cervical vertebrae and pleurodont teeth. Acteosaurus of Istria; anterior extremities distinctly shorter than the posterior pair; tail long. Vertebrae, like those of Dolichosaurus, with zygosphenes. Plioplatecarpus of the Upper Chalk of Holland has a slender interclavicle; the vertebrae are without zygosphenes, but those of the cervical region possess a downwardly directed long hypapophysial process with a separately ossified epiphysis.

Order II. MOSASAURI.

The two halves of the lower jaw are connected by ligament and are therefore movable as in Snakes. There are no sacral vertebrae, the pelvis having lost its connexion with the vertebral column. The formation of the limbs into paddles is more pronounced than in the Dolichosauri.

Mosasaurus, the chief genus, so called from Mosa, the Latin name of the river Maas, with several species from the Upper Cretaceous strata of the Netherlands, England, and North America. M. camperi, from Belgium, with a skull about 4 feet in length, armed with many large, curved, acrodont teeth. The vertebral column consists of about one hundred caudal and thirty-four precaudal vertebrae, of which seven are cervical, without zygosphenes. The total length of the type-specimen is estimated at 25 feet.

Platecarpus of North America and New Zealand, and various other North American genera, also contained species of large size.

Liodon.–Premaxilla without teeth, the others nearly smooth instead of being ridged. With a very wide distribution in the Chalk of Europe, North America, and New Zealand. L. haumuriensis of New Zealand seems to have been the giant amongst these monstrous marine creatures; its total length has been computed from imperfect fragments at 100 feet.

Clidastes, of the Upper Cretaceous of North America and Europe, although not so massive, comprises the most elongated forms. The cervical vertebrae possess long median hypapophyses with separate epiphyses; most of the vertebrae are much elongated and have well-developed zygosphenes. C. tortor had a skull nearly two feet and a half long.

CHAPTER XII

SAURIA–AUTOSAURI OR LACERTILIA–LIZARDS

Sub-Class XI.–SAURIA.

The Sauria, which comprise the Autosauri or Lacertilia in the wider sense and the Ophidia or Snakes, are the most recently developed groups of Reptiles. No fossils are known from strata earlier than those of the Cretaceous epoch. Their origin has probably to be looked for among the Prosauria, of which Sphenodon, cf. p. 294, is the only surviving member. The Sauria have attained their great development within the Tertiary period. They, both Autosauri and Ophidia, are now the two dominant Reptilian groups, and they have, so to speak, a future before them, being apparently still on the increase in numbers and species, but certainly not in size.

Order I. AUTOSAURI or LACERTILIA–LIZARDS.

Saurians which have the right and left halves of the mandibles connected by a sutural symphysis.

The overwhelming majority possess well-developed limbs, movable eyelids and cutaneous scales, covered by the mostly thin and horny epidermis. But there are many kinds of Autosauri, especially those belonging to the degraded, burrowing families, which have lost not only one or both pairs of limbs, but even the limb-girdles, while the eyes have become concealed beneath the skin, and in some cases the scales have been lost, or reduced to mere vestiges. Moreover in some of these burrowing and limbless forms the quadrate bones have become more or less immovable.

We divide the Autosauri into three sub-orders:–I. Geckones, p. 502; II. Lacertae, p. 513; III. Chamaeleontes, p. 567, with about 270, 1500, and 50 species respectively.

The Autosauri are of great interest, since they exhibit a great, almost endless variety in shape, size, and structure in direct adaptation to their surroundings. Most of these modifications are restricted to the external organs, or rather to those which come into direct contact with the outer world, namely the skin, the limbs, the tail, or the tongue. The majority of the Autosauri are terrestrial, but there are also semi-aquatic forms. There are climbing, swiftly running, and even flying forms, while others lead a subterranean life like earthworms. Most of them live on animal food, varying from tiny insects and worms to Birds and Mammals, while others live upon vegetable diet. According to this diet, the teeth and the whole digestive tract are modified. The intestine is relatively short in the carnivorous, long in the herbivorous species. But swiftness, the apparatus necessary for climbing, running, and digging, the mechanism of the tongue, the armament and the muscles of the jaws (hence modifications of the cranial arches, etc.), stand also in correlation with the kind of food and with the way in which it has to be procured.

A very interesting study of the influence of the climate and the nature of the country upon Reptiles has been made by Boettger^[147] with especial reference to the Transcaspian desert-region. The winter is there short, but very severe, and there is a considerable amount of snowfall, while the summer is intolerably hot. The spring arrives suddenly. Lilies and tulips, which have been asleep for nine or ten months, sprout towards the end of February, and a carpet of flowers covers the ground for a short time. Then everything shrivels up during the rainless and fierce heat of the summer, and the autumnal storms of dust and sand kill off the last remnants of vegetation. There are no trees, and even prickly shrubs are rare. Instead of broad leaves the plants have grass-like blades or needles. The little shrubs do not form coherent patches, but they are scattered about, and around the roots of each shrub the wind accumulates little mounds of sand and dust, a place of retreat for rodents, lizards, snakes, and even for the female tortoises. G. Kadde's "law of the steppe" is in full force;–there is little change of forms in a wide district, but all these forms are peculiar, and they congregate socially in great numbers. Most characteristic are those kinds of Geckos which, like Teratoscincus, cf. p. 507, have become inhabitants of sand instead of climbers of rocks and trees; various kinds of Phrynocephalus, cf. p. 521, and Varanus griseus; the four desert-species of Lacertidae are brownish-grey or sandy yellow, with conspicuous stripes or spots. Of snakes are to be mentioned Eryx jaculus, digging in the sand, and about ten other non-poisonous snakes. Tropidonotus is, of course, restricted to permanently watery places, where they can get frogs and fishes. Of poisonous snakes there is the Cobra and Echis arenicola. Of Amphibia only Bufo viridis and Rana esculenta var. ridibunda exist in suitable places, but there are neither Tree-frogs nor Newts.

Characteristic features of these inhabitants of the desert are the following:–

1. Velocity. The Lizards are slender. The Sand-snake, Tephrometopon, is whip-like; even the Cobra has a relatively narrower and longer tail than the Indian specimens, although the number of the vertebrae and of the scales is the same. All the desert-snakes are remarkable for the great number of their ventral shields, two hundred and more.

2. Hard, scaly covering, for instance in Agama, Echis, Gymnodactylus, Teratoscincus; the latter with its fish-like scales is exceptional among Geckos, resembling the likewise deserticolous Geckolepis and Homopholis of Africa.

3. Capacity for digging in the sand in order to escape great cold, or burning heat. All the Lizards and the Tortoise, Testudo horsfieldi, have strong claws. The snakes Typhlops and Eryx dig with their specially modified snouts, and their tails are very short and blunt. The Sand-viper, Echis, has the scales of the back arranged in very oblique rows, so that it can heap sand upon its body by wriggling, shaking, and up-and-down motions of the body. The Agamoid Phrynocephalus does this by means of lateral folds of the skin.

4. Arrangements for running on sand. The lizard Eremias has very large crural shields; Scapteira has the digits broadened out into shovels; others, e.g. Phrynocephalus and Teratoscincus, have long lateral fringes on the digits, a very rare arrangement among Geckos, occurring elsewhere among them only in Ptenopus and Stenodactylus, which are likewise inhabitants of the desert.

5. Protection against the everlasting, ubiquitous sand. In the digging species the nostrils are directed upwards instead of forwards; in most of the snakes they are protected by complicated valves, or they are reduced to small pin-holes. The eyes of Typhlops are overhung by the head-shields. In Agama and Phrynocephalus the margins of the lids are broadened into plates and are furnished with peculiar scales. In Teratoscincus the upper lid is enlarged. The lizard Mabuia has the lower lid much enlarged, with a transparent window in it, so that the eye can be closed without impeding sight, an arrangement carried to the extreme in Ablepharus, cf. p. 560. The ear-opening is either small, or protected by fringes of scales, or it is abolished, e.g. in Phrynocephalus.

6. Coloration. Pure green is quite absent, even in Bufo viridis and in Rana esculenta, since there is no green in that country, at least not of long duration. White, with grey and black spots, occurs only in the nocturnal Geckos. Yellow, brownish, reddish colours are common, in adaptation to the sand. The advantages of the carmine-red, and of the blue spots of Phrynocephalus, and the yellow or bright red under surface of its tail, are unknown. Striation is of frequent occurrence among the lizards and snakes, probably in adaptation to the dry grass heaped up around the scattered shrubs.

Concerning the various organic systems of the Autosauri only some of the more important features may here be mentioned.

Skeleton.–The vertebrae are procoelous, with the exception of most of the Geckones, in which they are amphicoelous. So-called intercentra, in the shape of unpaired nodules or wedges, persist between most of the cervical vertebrae. In the tail these wedges, the remnants of the basiventralia, are generally present, frequently in the shape of chevron-bones. Sometimes they fuse with the centra of the vertebrae; occasionally the axial or central portion of these basiventrals persists as a sort of fibrous disc, which may calcify separately, and is interposed between the caudal end of the centrum and the articulating knob. The caudal vertebrae of the Geckones and of most Lacertae are liable to break across, like those of Sphenodon. They are enabled to do this owing to a transverse split, which makes its appearance with the ossification of the vertebral bodies and extends later into and across the neural arch and the various lateral processes. The split is ultimately referable to a transverse septum of cartilage, wrongly called chordal cartilage, which develops in the shell of the body of the vertebra, destroys the chorda, and extends peripherally. The cells of this septum retain throughout life their juvenile quasi-embryonic character. When the tail is broken off–and this always happens at such a septum–the cells of the remaining half reproduce a new tail. The latter is, however, in reality a sham tail, since neither new centra nor arches, but only a non-segmented rod or tube of fibro-cartilage is produced by this process of regeneration. Reproduction of centra is precluded by the previous normal reduction of the chorda, around which alone proper bony centra could be formed. The regenerated tail is, however, invested with new muscles, and with skin, but the scales often differ considerably from those of the normal organ. Boulenger^[148] has found that the new or aberrant scaling is in some cases a reversion to an ancestral form. This is, for instance, the case in Pseudopus, and in the Tejoid genus Gymnophthalmus; to a certain extent also in Geckos and Skinks. On the other hand, Lacertidae, Gerrhosauridae, and also Anguidae reproduce a caudal scaling true to their type. Injured or broken-off tails are often reproduced double, or even trifid; sometimes an additional little tail grows out from an injured spot, anywhere on the side of the old remaining but mended tail.

The ribs of the trunk articulate by their capitula only, while the reduced tubercula are attached to their vertebrae by ligaments. In the tail the capitular portion is much reduced, while the tuberculum is much stronger and lies behind, no longer above, the capitulum, fusing sometimes directly with the centrum. The ribs of the poststernal region of Geckos and Chameleons are very long, and meet each other in the middle line, forming thin cartilaginous hoops.

The limbs are of the typical pentadactyloid type. The distal tarsalia are often fused with the metatarsals, so that the chief bending of the foot is effected by truly intertarsal joints. The greatest modification occurs in the foot of the Chameleons, in which the proximal tarsalia are reduced in number, and form a globe for the articulation with the tibia and fibula.

The shoulder-girdle and sternum much resemble that of Sphenodon in their completeness. The coracoids articulate with the sternum; the precoracoids and the basal parts of the scapulae often send out several processes towards those of the other side, so that several fenestrae are formed. The clavicles are complete, but are absent in the Chameleons. The interclavicle is mostly T-shaped. A presternum is absent, but the sternum proper is well developed, often forming a rhomboid plate, usually cartilaginous, often diverging backwards into xiphisternal processes.

The pelvis is attached to two vertebrae by means of several ribs. The ischium and pubis form symphyses. The pubis carries a well-developed lateral process, and the obturator-nerve pierces the shaft of the pubis. Epipubic and hypo-ischial cartilages are of frequent occurrence.

The hyoid apparatus consists of a median, styliform rod, which extends forwards into the tongue; it is often bifid behind. The unpaired piece carries two pairs of horns. The posterior of these, the first pair of branchial arches, extends backwards along the gullet, and is very long if the tongue is very slender and protractile. The anterior pair, the hyoid arches, consists of two pieces on either side, one short and directed forwards, the other long, connected with the former at a sharp angle and continued upwards to the sides of the skull, often in direct continuity with the columellar chain of the ear.

The modifications of the skull concern chiefly the composition of the temporal arches, see Figs. 55, M, N, O, p. 281. The quadrate bone is movable, but it has become fixed in various degraded families, where the skull shows a great reduction and concentration; the postorbital and temporal arches, the interorbital septum, and with it the columellae cranii are lost. The columella cranii of the Chameleons, which is generally stated to be absent, is really present, although in a much reduced state, and is partly imbedded in the interorbital septum. The occipital condyle has become bifid in Amphisbaenidae.

Burrowing and living in sand are often correlated with partial or complete reduction or loss of the limbs and their girdles. This loss of limbs is as a rule correlated with an elongation of the trunk, not always at the expense of the tail, which in such cases is much shortened. The vestiges of the hind-limbs come to lie as near the vent as possible. This reduction of the limbs occurred in several families which are not directly related to each other. Moreover, it does not occur in all the members of the family, not always in those of the same genus, and there is a considerable amount of individual variation. In most cases of reduction the fore-limbs disappear before, or are smaller than, the hind-limbs. In the Amphisbaenidae (cf. Chirotes, p. 566), and in the Tejidae the reverse takes place. In extreme cases the reduction is so complete that even the pectoral girdle has disappeared, leaving scarcely any trace, e.g. in Dibamus, p. 564.

The skin is normally covered with scales, which are formed by the cutis and have a horny epidermal coating. The latter, thin and transparent, is shed periodically, peeling off in flakes, except in Anguis and perhaps other snake-shaped creatures, which shed the skin in one piece. In the Amphisbaenidae the scales have practically disappeared. When well developed the scales are prominent, and imbricate or overlap with their free posterior edges; but in many cases the scales are not "scale-like" at all, only like little tubercles, which give the skin a granular appearance. Frequently, for instance in the Scincidae and Anguidae, all the scales contain "osteoderms," or ossified portions of the cutis, and encase the whole body and tail. In other families, e.g. Lacertidae, such osteoderms are restricted to the scales or shields on the head, where they come into contact and fuse with the underlying cranial bones, and moreover roof in the supratemporal fossa.

The skin of the Autosauri is entirely devoid of glands. The femoral and pre-anal pores of many families, occurring especially in the males, are probably not glands. They are arranged in rows on the under surface of the thighs and in front of the anal opening. Each of these organs perforates a scale and leads into a tubular invagination, which is lined with epidermal cells, the proliferation of which produces a horny yellowish débris, and this fills the tube and appears above the surface in the shape of a little cone. The use of this "excretion" is unknown; it is possibly hedonic.

Most Autosauri are capable of changing colour. In most of them this faculty is restricted to the assumption of paler or darker tints owing to the shifting of the colouring matter contained in the chromatophores. In others new, often vivid colours are the result. The mechanism is described in detail in the Chameleon on pp. 570 and 574.

Pigment is deposited either directly in the upper strata of the cutis, just below the Malpighian layer, or it is contained in chromatophores. The latter are imbedded in the deeper layers of the cutis, and send out movable contractile processes, in which their pigmented protoplasm is conveyed towards or away from the surface. The only colours available are black, red, yellow, and white, with their combinations of grey and brown. The white pigment consists of guanin-salts. Blue and green are structural colours, not due to pigment. The same can no longer be said of the Ophidia, since Boulenger has observed accidentally that green Tree-snakes (e.g. Dryophis) give the alcohol in which they are kept the colour of green Chartreuse.

Digestive organs.–The tongue is very variably developed, and affords good taxonomic characters. It is always furnished with many tactile, or with gustatory, corpuscles. When the tongue is very long and narrow it is generally forked, and in these cases, for instance in the Varanidae, is almost entirely used as a sensory organ. In others, especially where it is broad, it assists in catching the food, and in the Chameleons it has attained a most elaborate development (see p. 569).

Salivary glands are restricted to labial glands. In Heloderma those of the lower jaw are transformed into poison-glands, an analogy to what prevails in the poisonous snakes. The intestinal canal is longest in the herbivorous forms; the rectum sometimes possesses a short blind sac or caecum.

The cloaca of the Sauria is somewhat modified; instead of the Coprodaeum, Urodaeum, and Proctodaeum forming three successive chambers, the urodaeum is practically reduced to its dorsal half, forming a dorsal recess between the two other chambers. The Coprodaeum is constricted into several successive chambers, and is always well shut off from the urodaeum by a strong sphincter. The urodaeum receives the urinary excretions, which are mostly chalky white and are rather consistent instead of being fluid. The right and left oviducts also open into it. The vasa deferentia open into the dorso-lateral portions of the walls of the urodaeum, but the sperma is conducted by folds of the lining of this chamber towards the bases of the copulatory organs, which, although arising from the lateral and posterior corners of the cloaca, where uro- and procto-daeum meet, are stowed away outside the cloaca. These organs are always paired. The proctodaeum or outermost cloacal chamber is shallow. Its inner opening is round and is furnished with a sphincter, but it is surrounded and covered by lips of the outer skin, which form a transverse slit. This is due to the peculiar arrangement of the copulatory organs.

Each organ consists of a tube of erectile tissue, and can be everted like the finger of a glove. To the apex of the tube is attached a long retractor muscle, which arises from the ventro-lateral surfaces of the caudal vertebrae and extends a considerable distance back. When at rest and withdrawn the organs form slight conical, longitudinal swellings on either side of the root of the tail, an external feature by which male specimens can generally be distinguished. Only one organ is inserted at one time.

The majority of Autosauri lay eggs, surrounded by a white or yellowish shell, which is either hard, for instance in Geckos, or parchment-like, e.g. in Chameleons, in Lacerta viridis and L. agilis, and in L. vivipara. Eggs with a thin and soft shell sometimes exhibit the paradoxical feature of increasing in size after they have been laid. This is explained by the growth of the embryo, which stretches the shell and does not merely live upon the white and yellow contents of the egg itself, but also takes in air and moisture. Many Lizards do not lay their eggs until they contain ripe embryos, which burst the shell shortly after deposition. Some, for instance Lacerta vivipara, Anguis fragilis, and Chamaeleo pumilus, are practically viviparous. The embryos, especially those which are enclosed in hard-shelled eggs, are provided with a sharp, calcareous "egg-tooth" on the top of the snout.

The lungs are thin-walled sacs, sometimes provided with lateral ex-sacculations, and these reach their greatest development in the Chameleons. The breathing is effected by the motion of the ribs. Inflatable sacs on the throat, or on the sides of the neck, for ornamental or sexual purposes, occur in various families. The lungs of much-elongated, snake-shaped Lizards are generally asymmetrical; the right being reduced in Amphisbaenidae; the left in other cases.

Several Autosauri, for instance the Geckos, Psammodromus, and various other Lacertidae have a weak voice.

The Fat-bodies are mysterious organs which are situated beneath the skin, and extend from the inguinal region forwards along the ventral sides of the belly. They are often of considerable dimensions; largest in the spring, in both sexes, at the time of propagation. Their colour is greyish-white or yellow, owing to the great accumulation of fat in the meshes of the connective tissue which composes the frame-work of these organs. An artery enters them, breaks up into capillaries, and these combine to form an efferent vein. After the time of propagation these organs are reduced to grey or reddish flaps, consisting mainly of very vascular connective tissue. G. W. Butler^[149] has written a long paper on their morphology. The same author^[150] has investigated the "sub-divisions of the body-cavity in Lizards, Crocodiles, and Birds," with reference to peritoneal diaphragmatic structures.

The geographical distribution of the Autosauri teaches few, but important lessons. We have to restrict ourselves to the principal families, leaving out those which are small and have a limited distribution; also those which, like the few Anelytropidae in Africa and in Mexico, are not natural groups.

The Geckones, which are probably the oldest of modern Autosauri, are practically cosmopolitan, being absent only in the cold and in the cooler temperate regions. They are common even in Oceanic Islands, for instance in New Zealand and in the Sandwich Islands. Although not at all aquatic, they are particularly fit to be transported accidentally on or in the trunks of floating trees, to which they cling firmly, and they can exist without food for months. I once received a little South American Gecko in perfect health from a grocer, who found it in a well-closed wooden box containing canned meat, two months after delivery of the box in Cambridge.

The Scincidae, likewise an old family, are equally cosmopolitan, but although many exist in the islands of the Pacific a few only occur in New Zealand. Many of the genera have a very wide distribution; for instance, Lygosoma, with its one hundred and sixty or more species, occurs in the Australian and Palaeotropical regions, and also in North and Central America, not extending, however, into South America. Mabuia, with more than sixty species, occurs in the Palaeotropical and the Neotropical regions. Whether these and other widely-distributed genera are all natural is another question.

The Agamidae, Varanidae, Lacertidae, and the Chamaeleontes are restricted to the Old World. The Agamidae and Varanidae have the widest distribution, occurring in the whole of the Old World with the notable exception of Madagascar and New Zealand. The Lacertidae are Palaearctic and Palaeotropical, being however absent in Madagascar, and, broadly speaking, not extending eastwards beyond Wallace's line. It is a most suggestive fact that most of those families of Reptiles, and even of other Vertebrates which have a wide distribution and are apparently debarred from transgressing Wallace's line, are also absent from Madagascar.

The Chameleons are essentially African, with their centre of greatest abundance and development in Madagascar, only one or two species occurring in Socotra, Southern Arabia, and in Ceylon and Southern India. Since they also exist, Ch. sechellensis, on various islands in the Indian Ocean, for instance in Mauritius and the Seychelles, the Chameleons are perhaps an indication of the former existence of a direct land-connexion between Southern India and Southern Africa.

The Iguanidae are essentially American, with the remarkable exceptions of Chalarodon and Hoplurus in Madagascar, and Brachylophus in the Fiji and Friendly Islands. This peculiar distribution finds some analogies in that of Dendrobatinae (p. 272), certain Boinae (p. 601), and Centetes and Solenodon among Insectivora. An Iguana (I. europaea) has, however, been described from the Eocene of France and England. The supposed relationship of the Iguanidae with the Agamidae makes the problem only more puzzling, since Agamidae are absent in Madagascar. If we have recourse to the Zonuridae, which are confined to Africa and Madagascar, and are supposed to be intermediate between Anguidae and Iguanidae, then we may have ultimately to conclude that the Malagasy Iguanoid genera and the American Iguanidae are a case of convergent evolution.

The Amphisbaenidae are distributed over America, including the West Indies, Africa exclusive of Madagascar, and the Mediterranean countries. This is very puzzling, considering that these subterranean, helpless creatures positively cannot travel. Boulenger regards them "as a degraded type of the Tejidae, with which they are to some extent connected by Chalcides and its allies," i.e. genera with reduced limbs, cf. p. 562.

However, this supposed relationship with a strictly American family does not explain the occurrence of Amphisbaenidae in Africa. Either they are not a natural group, or they had, as already degraded, limbless creatures, a much wider range; and this would imply their being a very old family, perhaps as old as we suppose the Coecilians to be.

Anguidae occur in North and South America, in Europe and the Mediterranean parts of North Africa, and in Trans-Gangetic India. Their older relations, the Zonuridae, inhabit Africa and Madagascar.

Madagascar is consequently devoid of Agamidae, Varanidae, Lacertidae, Anguidae, and Amphisbaenidae, while it possesses, besides the cosmopolitan Scincidae and Geckones, only Chameleons, Gerrhosauridae, and Zonuridae,–all three essentially African families,–and a few Iguanidae. This means that the Autosaurian fauna of Madagascar is intimately related to that of Africa, and that it possesses only old families so far as Sauria are concerned. But since this great island was separated from its continent not earlier than in Mid-Tertiary times, it follows that most of these "old" families are comparatively recent.

Australia possesses only Agamidae and Varanidae besides the ubiquitous Geckos and Skinks. Besides the latter two families it has nothing in common either with Madagascar (an analogy with the Anura) or with America. The Autosauri consequently do not support the idea of a Notogaea, cf. p. 74. This again indicates the comparatively recent age of Autosaurian families. The marked difference which exists between the Old and the New World points to the same conclusion. On the other hand, the Autosauri support the idea that the Palaeotropical region is but the tropical and therefore richer continuation of the now impoverished Palaearctic sub-region.

Sub-Order 1. Geckones.–The typical Geckos are characterised as follows. Four-footed Autosauri with amphicoelous vertebrae; skull without bony temporal arches; clavicles dilated and with a perforation near the ventral end; parietal bones separate; eyes (with few exceptions) without movable lids; pleurodont; tongue fleshy and broad, slightly nicked anteriorly, and capable of protrusion.

This definition does not apply to a few forms. In the Eublepharinae the vertebrae have advanced to the procoelous condition, and the parietals are fused together, while the eyes are provided with typical, movable lids. In the Uroplatinae the clavicles are not dilated, and the nasals are fused into one bone. The Geckos seem to be not only a very independent but also a very old branch of Saurians. Although fossil representatives are unknown, the resemblance of their vertebrae to those of the Palaeozoic Microsauri is at least remarkable. They are now practically cosmopolitan within the warmer zones, being found in abundance in all intertropical countries and islands, even in New Zealand. About two hundred and seventy species are known, which have been subdivided into about fifty genera. The generic differences are trivial with few exceptions, and refer mostly to the structure of the digits.

The more important features of the vertebral column are the absence of axial joints and the persistence and life-long growth of the chorda dorsalis. Each vertebral centrum consists of a cartilaginous tube, more or less calcified or ossified, with a narrow waist and a cartilaginous septum in the middle. In the tail this septum, which is only slightly invaded by ossification, coincides exactly with the line of transverse division of the vertebrae into an anterior and a posterior half. This is the level where the tail breaks off and whence it is renewed. Between every two successive centra lies an intercentrum, broadest ventrally, crescent- or wedge-shaped. Dorsally it is continued as fibro-cartilage, and the whole ring acts as an articular pad instead of the joint. Chevron-bones are common in the tail.

The ribs are bifurcated, but the tubercular portion is frequently reduced. The post-thoracic ribs are usually very slender, and so long that they meet each other in the middle line, in this case bearing an extraordinary resemblance to the so-called "abdominal ribs" of other reptiles.

The bony frame of the skull is slender. There is a complete absence of bony arches spanning over the temporal fossae, or bordering the orbit, which is open posteriorly. The upper jaw, owing to the slender and flexible nature of the respective bones, is movable upon the rest of the skull; in this respect not unlike the upper jaw of a duck. The dentition is pleurodont and the teeth are minute. The eyes of the typical Geckos are peculiar. They are covered with an absolutely transparent skin, shaped like a watch-glass, beneath which the eye moves freely, while the true upper and lower lids are reduced to tiny folds. The covering "watch-glass" is probably a modification of the nictitating membrane. In the Eublepharinae, however, and in the few species of the Geckonine genera Aelurosaurus of Borneo and Australia, and Ptenopus of South Africa, the upper and lower lids are present and movable. The pupil contracts mostly into a vertical slit, except in the few diurnal kinds, e.g. Phelsuma, of the islands in the Indian Ocean, and the African Lygodactylus.

Another peculiarity of at least many Geckos is the extraordinary development of the endo-lymphatic sacs of the ear, which, being filled with the chalk-like otoconia or otolithic crystals, perforate the skull, and are stowed away in the shape of a pair of large bags behind the ears, or on the sides of the neck.

The skin exhibits considerable variety. It is mostly soft above, with little granular tubercles, sometimes containing small dermal ossifications or calcifications. The latter are most developed on the head, where they occasionally fuse with the underlying bones. A few species of Tarentola possess supra-orbital bones, independent remnants of such osteoderms. The ventral surface is generally covered with small imbricating scales, but in some genera, e.g. Homopholis, such scales occur also on the dorsal surface, reaching their highest development in Teratoscincus (p. 507). In a few forms, notably in Ptychozoon (p. 512), the skin of the sides of the body and tail is produced into a series of lobes and flaps, the object of which seems to assist adhesion. Many, perhaps the majority of Geckos, have adhesive digits, by means of which some kinds are enabled to climb absolutely smooth and vertical surfaces, for instance a window-pane; or, what is more startling, they run along the smooth, white-washed ceiling, back downwards. The apparatus is complicated in its minute detail, but is very simple in principle. The adhesion is effected neither by sticky matter, nor in the way described in the Anura (p. 187), but by small and numerous vacua. The under surface of each digit is furnished with many transverse lamellae. The pressing down of the foot upon a smooth surface causes the lamellae to spread asunder and to drive out the air; partial retraction lets them return to their original position by virtue of their elasticity; and little vacua are produced. Each lamella is further beset with tiny hair-like excrescences, which secure adpression to even the slightest irregularity of surface and at the same time enhance the elasticity of the pads. The arrangement of the lamellae and pads differs much in the various genera. For instance, the lamellae are either broad and entire, or they are divided into two parallel rows, with or without lateral hairy fringes; or the under surface of the digits is granular, but strongly fringed; or the lamellae are restricted to the dilated tips of the digits, etc. The fingers and toes are mostly furnished with sharp, curved claws, and these are in many cases retractile between some of the lamellae, or into a special sheath. Those Geckos which live on sandy, barren ground are as a rule devoid of adhesive pads, the digits being narrow. The typically padded, adhesive digits cause a peculiar sensation when a Gecko hangs on to one's finger, and this feeling has perhaps given rise to the erroneous notion of stickiness.