Fourthly, in all the Serpents the teeth are anchylosed with the jaw, and are never sunk into distinct sockets or alveoli.

The Ophidia are usually classified in accordance with the characters of their dental apparatus, and may be divided as follows, some minor groups being omitted:-(1.) The Viperina comprise the most typical of the venomous Snakes (Venenosa), and include the common Vipers (Viperida) and the Rattlesnakes (Crotalida), the former being mostly confined to the Old World, whilst the latter are mainly American. The common Viper (Pelias berus) occurs abundantly in England and Scotland, and is capable of inflicting a severe and even dangerous bite, though it is doubtful if fatal effects ever follow except in the case of children or subjects previously debilitated.

Fig. 277.-The Naja Haje, a venomous Colubrine Snake.

The true Rattlesnakes (Crotalus) are exclusively natives of America, and they are highly poisonous. The extremity of the tail in the true Rattlesnake (Crotalus horridus, fig. 273) is furnished with a series of horny epidermic cells of an undulated pyramidal shape, articulated one within the other. constituting an appendage which is known as the "rattle." Before striking its prey, the Rattlesnake throws itself into a

coil, and shakes its rattle, as it does also when alarmed. According to Professor Shaler, the use of the rattle is to imitate the note of the Cicada, and thus to attract birds which prey upon this insect; but its function is more probably sexual. The Indian snakes belonging to the genus Trimeresurus, and some other less important forms, are also placed amongst the Crotalida. The head of the Viperine Snakes (figs. 274, 278) is broad, somewhat triangular in shape, broadest in its middle, and showing a very distinct line of demarcation between the head and neck. The head, also, is usually covered with small scales, rarely interspersed with larger plates or "scuta" (fig. 278). Other well-known members of this group are the Death Adder (Acanthophis tortor) of Australia, the Horned Viper (Cerastes) of Africa, and the Puff Adder (Clotho arietans) of the Cape of Good Hope.

(2.) The Elapina are poisonous snakes, in which the poisonfangs are permanently fixed and erect, and have smaller solid teeth behind them. The head is shield-shaped, and not much wider than the body. This group comprises some of the most deadly of all the Serpents, one of the best known being the Hooded Snake or Cobra di Capello (Naja tripudians), which is commonly found in Hindostan, and is the snake usually carried about by the Indian snake-charmers. It varies from two to six feet in length, and the neck can be extensively dilated, covering the head like a hood. A nearly allied species is the Naja Haje (fig. 277) of Egypt. The genus Bungarus, including the deadly "Kerait" (B. cæruleus) of India, is nearly allied to Naja, but the neck is not dilatable. America has representatives of this family in the beautifully marked Coralsnakes and Harlequin - snakes (Elaps), and the Australian region is also not without them (Hoplocephalus, &c.)

(3.) The Hydrophida comprise aquatic Ophidians which have the tail vertically compressed and broadened out. They are found principally in the Indian and Chinese seas, often frequenting the mouths of rivers, though sometimes ranging far from land. They are extremely poisonous, and swim with great ease and rapidity.

(4.) The Colubrina comprise an immense number of altogether innocuous snakes, in which the superior maxillæ are provided with solid teeth only, and there are no fangs. An excellent example of this group is the common Ringed Snake (Coluber or Tropidonotus natrix) of Britain, a perfectly harmless animal, which is commonly found in damp situations, and which lives mainly upon frogs. Closely allied to this is the Black Snake (Bascanion constrictor) of North America, which

attains a length of from three to five feet, but is perfectly harmless, so far as man is concerned.

(5.) The Pythonina comprise the well-known serpents termed the Boas, Pythons, Anacondas, and Rock-snakes. The members of this group are the largest of all living snakes, attaining a length of certainly over twenty feet. Their bite is perfectly harmless, but they are nevertheless highly dangerous and destructive animals, owing to their great size and enormous muscular power. They seize their prey and coil themselves round it in numerous folds, by tightening which they gradually reduce their victim to the condition of a shapeless bolus, fit to be swallowed. In this way a good-sized Python or Boa will certainly dispose of an animal as large as a sheep or goat, and it is asserted that even human beings may be thus devoured by large individuals of the family. The Boas and Pythons occur in both the Old and New Worlds, the Pythons, however, all belonging to the Old World, and they are amongst the most formidable of all living Ophidians. They possess rudimentary hind-limbs terminating in horny anal spurs, which co-operate with the prehensile tail in enabling the animal to suspend itself from trees. In all, also, the dental apparatus is extremely powerful, giving a firm hold for the constriction of the prey.

(6.) The Typhlopida constitute an aberrant group of snakes, distinguished from the typical Ophidians by the comparative narrowness of their gape. They are found in both the Old and New Worlds, in warm latitudes, are possessed of quite rudimentary eyes, and burrow in the ground. Some of them are of very small size, and somewhat resemble earthworms. Nearly allied to the genus Typhlops itself is the Indian Uropeltis, which is also subterranean in its habits.

A good general character by means of which the poisonous Viperine Snakes may be distinguished from the harmless Colubrine forms is in the shape and armature of the head. In the Viperina, as before said, the head (figs. 274, 278) is triangular, broadest behind, and separated from the neck by a more or less marked diminution in the diameter of this latter part. The scales, too, which cover the head, are of small size. In the Colubrine snakes, on the other hand, the head is not markedly triangu lar, and gradually tapers off into the neck, whilst the upper surface of the head is usually covered with large shield-like plates or "scuta" (fig. 278, A).

DISTRIBUTION OF OPHIDIA IN TIME.-The Ophidia are not known to occur in any Palæozoic or Mesozoic deposit. The earliest known traces of any serpent are in the Lower Kainozoic rocks, the oldest being the Palæophis toliapicus of the London Clay of Sheppey. The nearly-allied Palæophis typhæus

of the Eocene beds of Bracklesham appears to have been a Boa-constrictor-like snake of about twenty feet in length.

Fig. 278.-A, Head of Colubrine Snake (Coluber natrix); B, Head of Viperine Snake (Pelias berus); C, Head of Blind-worm (Anguis fragilis), one of the serpentiform Lizards. (After Bell.)

Other species of Palæophis have been described from the Tertiary rocks of the United States, and the genus Dinophis has been formed for the reception of another gigantic constricting Serpent from the same formation. In some of the later deposits have been found the poison-fangs of a venomous snake. Upon the whole, however, the snakes must be looked upon as a comparatively modern group, and not as one of any great geological antiquity.

CHAPTER LXII.

LACERTILIA AND CROCODILIA.

ORDER III. LACERTILIA.-The third order of Reptiles is that of the Lacertilia, comprising all those animals which are commonly known as Lizards, together with some serpentiform animals, such as the Blind-worms. The Lacertilia are distin

guished by the following characters :

As a general rule, there are two pairs of well-developed limbs, but there may be only one pair, or all the limbs may be absent.

A scapular arch is always present, whatever the condition of the limbs may be. An exoskeleton, in the form of horny scales like those of the Snakes, is almost always present. The vertebra of the dorsal region are procælous or concave in front, rarely amphi cœlous or concave at both ends. There is a single transverse process at each side, and the heads of the ribs are simple and undivided. There is either no sacrum, or the sacral vertebræ rarely exceed two in number. The teeth are not lodged in dis tinct sockets (some extinct forms constituting an exception to this statement). The eyes are generally furnished with movable eyelids. The heart consists of two auricles and a ventricle, the latter partially divided by an incomplete partition. There is a urinary bladder, and the aperture of the cloaca is transverse.

As a general rule, the animals included under this order have four well-developed legs (fig. 279), and would therefore

Fig. 279.-Iguana.

be popularly called "Lizards." In some (Chirotes) there are no hind-feet; in some (Bipes) the fore-limbs are wanting; and others (Anguis, Pseudopus, and Amphisbæna) are entirely destitute of limbs, thus coming closely to resemble the true Snakes or Ophidians in external appearance. These serpentiform Lizards, however, can be distinguished from the true Snakes, amongst other characters, more especially by the structure of the jaws. In the Snakes, as before said, the two rami of the lower jaw are loosely united in front by ligaments and muscles, and are attached behind to a movable quadrate bone, which is in turn connected with a movable squamosal, this giving an enormous width of gape to these animals. In the Lizards, however, even in those most like the Snakes, the halves of the lower jaw are firmly united to one another in front; and though the quadrate bone is usually more or less movable, the jaws can in no case be separated to anything like the extent that characterises the Ophidia.