It is hardly possible, with our present knowledge, to speak very positively as to the exact range of the Lacertilia in time. This uncertainty arises from two causes, - firstly, that there is some doubt as to the exact age of some deposits which have yielded Lacertilian remains; and secondly, that the affinities of some extinct Reptiles are a matter of considerable question. Upon the whole, the oldest known Lacertilian would appear to be the Protorosaurus of the Middle Permian rocks; though good authorities have placed this form in the Crocodilian group of the Thecodontia. Protorosaurus attained a length of between five or six feet, and differs from all existing Lizards in having its teeth implanted in distinct sockets - this being a Crocodilian character. In other respects, the Permian reptile approximates closely to the living Monitors (Varanidae), and its slightly cupped vertebrae would lead to the belief that it was aquatic in its habits. Both pairs of limbs were present, both pentadactylous, and constructed on the type of the limbs of the typical Lizards.

In rocks known or supposed to be of Triassic age, several Lacertilian reptiles have been discovered, of which the most important are Telerpeton, Hyperodapedon, and Phynchosaurus, of which the last is sometimes referred to the group of the Anomodontia, to be subsequently spoken of.

In the Jurassic period, the remains of Lacertilians are not unknown, but call for little special notice. Several forms of little importance have been described from the Middle Oolites. In the fresh-water strata of the Purbeck series (Upper Oolites), occur the remains which have been referred to the genera Nuthetes, Macellodon, Saurillus, and Echinodon. These are, perhaps, the first traces in the stratified series of remains, the affinities of which to the typical Lacertidae cannot be disputed.

In the Cretaceous rocks occur the singular Lacertilians which form the group of the "Mosasauroids." These remarkable Reptiles were of gigantic size, Mosasaurus princeps being believed to have attained the enormous length of not less than seventy-five feet. The teeth of these reptiles are long, conical, and slightly curved; but they are anchylosed to the jaw, and are not sunk into distinct sockets as in the living Crocodiles. The vertebrae are procoelous. From the shortness of the humerus, and the indications that the vertebral column was unusually flexible, and that the tail was laterally compressed, it was early conjectured that the Mosasauroids were marine and aquatic in their habits. This conjecture has been raised to the rank of a certainty by the discovery that the fore and hind limbs of the Mosasauroids were in the form of fin-like paddles, resembling the flippers of whales in general structure, and in having the digits distinct and only conjoined by integument (fig. 308). There can therefore be no doubt that Mosasaurus - like the living Amblyrhynchus - was aquatic in its habits, and frequented the sea-shore, coming, in fact, only occasionally to the land. Professor Marsh has also recently shown that some species possess bony dermal scutes, thus rendering their Lacertilian affinities somewhat dubious.

Though possessing certain aberrant characters, it seems best in the meanwhile to regard the Mosasauridae (= the Pythonomor-pha of Cope) as an extinct group of the Lacertilia.

Order IV. Crocodilia. -The last and highest order of the living Reptilia is that of the Crocodilia, including the living Crocodiles, Alligators, and Gav-ials, and characterised by the following peculiarities :

The body is covered with an outer epidermic exoskeleton composed of horny scales, and an inner dermal exoskeleton consisting of transverse rows of squared bony plates or scutes, which may be confined to the dorsal surface alone, or may exist on the ventral surface as well, and which are disposed on the back of the neck into groups of different form and number in certain species. The bones of the skull and face are firmly united together, and the two halves or rami of the lower jaw are united in front by a suture. There is a single row of teeth, which are implanted in distinct sockets, and hollowed at the base for the germs of the new teeth, by which they are successively pushed out and replaced during the life of the animal. The centra of the dorsal vertebra in all living Crocodilia are procoelous or concave in front, but in the extinct forms they may be either amphicoelous (concave at both ends) or opisthocoelous (concave behind). The vertebral ends of the anterior trunk-ribs are bifurcate. There are two sacral vertebrae. The cervical vertebrae have small ribs (hence the difficulty experienced by the animal in turning quickly); and there are generally false abdominal ribs produced by the ossification of the tendinous intersections of the recti muscles. There are no clavicles. The sternum is rhomboidal and cartilaginous, sending backwards a pair of "xiphoid" processes. On its face, anteriorly, is a bony "in-terclavicle," while a scapula and coracoid exist on each side. The heart consists of four completely distinct and separate cavities, two auricles, and two ventricles; the ventricular septum - as in no other Reptiles - being 'complete. The right and left aorta, however - or, in other words, the pulmonary artery and systemic aorta - are connected together close to their origin by a small aperture (foramen Panizzae), so that the two sides of the heart communicate with one another. The aperture of the cloaca is longitudinal, and not transverse as in the Lizards. All the four limbs are present, the anterior ones being pentadactylous, the posterior tetradactylous. All are oviparous.

Fig. 308.   Right anterior paddle of Les tosaurus simus, one twelfth of the natural size. (After Marsh.) a Scapula ; b Coracoid ; c Humerus ; d Radius ; e Ulna.

Fig. 308. - Right anterior paddle of Les-tosaurus simus, one-twelfth of the natural size. (After Marsh.) a Scapula ; b Coracoid ; c Humerus ; d Radius ; e Ulna.

The chief points by which the Crocodiles are distinguished from their near allies the Lacertilians, are the possession of a partial bony dermal exoskeleton in addition to the ordinary epidermic covering of scales, the lodgment of the teeth in distinct sockets, and the fact that the mixture of venous and arterial blood, which is so characteristic of Reptiles, takes place, not in the heart itself, but in its immediate neighbourhood, by a communication between the pulmonary artery and aorta directly after their origin.

Fig. 309.   A, Head and anterior portion of the body of Crocodilus pondicerianus; B, Hind foot of the same. (After Gunther.)

Fig. 309. - A, Head and anterior portion of the body of Crocodilus pondicerianus; B, Hind-foot of the same. (After Gunther.)

When the exoskeleton is complete (as in Caiman), it consists of transverse rows of quadrate bony plates disposed so as to form a distinct dorsal and ventral shield, which are separated by soft skin in the region of the trunk, but become confluent in the tail. All the scutes of one row are united by suture, and successive rows usually movably overlap one another.

The only other points about the Crocodiles which require special notice are, that the eyes are protected by movable eye-lids; the ear is covered by a movable ear-lid; the nasal cavities open in front by a single nostril, and are shut off from the cavity of the mouth, but open far back into the cavity of the pharynx; and lastly, the tongue is large and fleshy, and is immovably attached to the bottom of the mouth. (Hence the belief of the ancients that the Crocodile had no tongue). The tail is long and compressed, with two rows of keeled plates, which unite about its middle to form a single crest, which is continued to its extremity. The feet are palmate or semi-palmate, and only the three inner toes on each foot possess claws. The eyes possess three distinct lids, and there are two glands under the throat secreting a musky substance.

The Crocodilia abound in the fresh waters of hot countries, and are the largest of all living Reptiles, not uncommonly attaining the length of twenty feet or upwards.

They are divided by Owen into three sub-orders, according to the shape of the dorsal vertebrae, termed the Procoelia, Amphicoelia, and Opistho-coelia.