hood, by a communication between the pulmonary artery and aorta directly after their origin.

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

Fig. 287.-A, Head and anterior portion of the body of Crocodilus pondicerianus; B, Hind-foot of the same. (After Günther.)

(Hence the

immovably attached to the bottom of the mouth. 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 semipalmate, 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 a 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 Procalia, Amphicalia, and Opistho

cœlia.

Sub-order 1. Procalia. -In this sub-order are all the living members of the Crocodilia, distinguished by having the bodies of the dorsal vertebræ concave in front (procoelous). Three distinct types may be distinguished amongst the living Crocodilia. The Gavial is distinguished by its elongated snout, at the extremity of which the nostril is placed, and by the fact that the teeth are pretty nearly equal in size and similar in form in the two jaws. In the true Crocodiles (fig. 288) the fourth tooth in the lower jaw is larger than the others, and forms a canine tooth, which is received into a notch excavated in the side of the alveolar border of the upper jaw, so that it is visible externally when the mouth is closed. In the Caimans or Alligators the same tooth in the lower jaw forms a canine, but it is received into a pit in the palatal surface of the upper jaw, where it is entirely concealed when the mouth is shut. The Crocodiles have the hind-legs bordered by a toothed fringe, and the toes completely united by membrane. They are essentially natives of fresh water, but sometimes frequent the mouths of rivers. They occur chiefly in Asia and Africa, but species are found in some of the West Indian Islands. The Alligators have the hindlegs simply rounded, and the feet not completely webbed. They are essentially aquatic, and are voracious animals, living upon fish or Mammals. The best-known species are the Alligator of the southern United States (A. Mississippiensis), the Caiman (A. palpebrosus) of Surinam and Guiana, and the "Jacaré" or Spectacled Alligator (A. sclerops) of Brazil. The Gavials inhabit fresh waters, and appear to be exclusively confined to

the Ganges and other large rivers of India. The Gangetic form (Gavialis Gangeticus), in spite of its numerous pointed teeth, is not so highly carnivorous as the true Crocodiles.

True procolian Crocodiles occur for the first time in the Greensand (Cretaceous series) of North America. In Europe, however, the earliest remains of procœlian Crocodiles are from the Lower Tertiary rocks (Eocene). It is a curious fact that in the Eocene rocks of the south-west of England, there occur fossil remains of all the three living types of the Crocdilia-namely, the Gavials, true Crocodiles, and Alligators; though at the present day these forms are all geographically restricted in their range, and are very partially associated together.

Sub-order 2. Amphicalia.-The Amphicœlian Crocodiles, with bicon

cave vertebræ, are entirely extinct. They have but a limited geological range, extending only from the Trias to the Chalk inclusive, and being therefore strictly Mesozoic. The biconcave vertebræ show a decided approach to the structure of the backbone in fishes; and as the rocks in which they occur are marine, there can be little doubt but that these Crocodiles were, in the majority of cases at any rate, marine. The most important genera belonging to this order are Teleosaurus, Belodon, Stagonolepis, Steneosaurus, Dakosaurus, Makrospondylus, and Suchosaurus, the last being from the fresh-water deposits of the Wealden (Cretaceous).

Sub-order 3. Opisthocalia.-The sub-order of the Opisthocalian Crocodiles, including those forms in which the anterior trunk vertebræ are concave behind, is one which can be only provisionally retained. Professor Owen includes in this section the two genera Streptospondylus and Cetiosaurus; but the latter is referable to the Deinosauria, and will be treated of when that order is considered. The genus Streptospondylus has been founded on vertebræ obtained from the Oolitic and Wealden formations; but there are doubts as to the true position of the reptile to which these belonged.

CHAPTER LXIII.

EXTINCT ORDERS OF REPTILES.

It remains now to consider briefly the leading characters of six wholly extinct orders of Reptiles, the peculiarities of which are very extraordinary, and are such as are exhibited by no living forms.

ORDER V. ICHTHYOPTERYGIA, Owen ( = Ichthyosauria, Huxley). The gigantic Saurians forming this order were distinguished by the following characters :

The body was fish-like, without any distinct neck, and probably covered with a smooth or wrinkled skin, no horny or bony exoskeleton having been ever discovered. The vertebra were numerous, deeply biconcave or amphicælous, and having the neural arches united to the centra by a distinct suture. The anterior trunk-ribs possess bifurcate heads. There is no sacrum, and no sternal ribs or sternum, but clavicles were present as well as an interclavicle (episternum); and false ribs were developed in the walls of the abdomen. The skull had enormous orbits separated by a septum, and an elongated snout. The eye-ball was protected by a ring of bony plates in the sclerotic. The teeth were not lodged in distinct sockets, but in a common alveolar groove. The fore and hind limbs were converted into swimming paddles, the ordinary number of digits (five) remaining recognisable, but the phalanges being greatly increased in number, and marginal ossicles being added as well. A vertical caudal fin was in all probability present.

The order Ichthyopterygia includes only the gigantic and fish-like Ichthyosauri (fig. 289), all exclusively Mesozoic, and

abounding in the Lias, Oolites, and Chalk, but especially characteristic of the Lias. If, however, the Eosaurus Acadiensis (Marsh) of the Coal-measures of Nova Scotia be rightly re

Fig. 290.-A, Pectoral arch and fore-limbs of Ichthyosaurus: a Interclavicle; & Clavicles; cc Scapula; dd Coracoids; e Humerus; f Radius; g Ulna. (Somewhat altered from Huxley). B, Pelvis of Ichthyosaurus: Pubis ; il Ilium; is

Ischium.

ferred to this order, then the Ichthyopterygia date from the Carboniferous period. There is no doubt whatever but that the Ichthyosauri were essentially marine animals, and they have been often included with the next order (Sauropterygia) in a common group, under the name of Enaliosauria or Sealizards.

In the biconcave vertebræ and probable presence of a vertical tail-fin, the Ichthyosaurus approaches the true fishes. There is, however, no doubt as to the fact that the animal was strictly an air-breather, and its reptilian characters cannot be questioned, at the same time that the conformation of the limbs is decidedly Cetacean in many respects. Much has been

gathered from various sources as to the habits of the Ichthyosaurus, and its history is one of great interest. From the researches of Buckland, Conybeare, and Owen, the following facts appear to be pretty well established :-That the Ichthyosauri kept chiefly to open waters may be inferred from their strong and well-developed swimming-apparatus. That they occasionally had recourse to the shore, and crawled upon the beach, may be safely inferred from the presence of a strong and well-developed bony arch, supporting the fore-limbs, and closely resembling in structure the scapular arch of the Ornithorhynchus or Duck-mole of Australia. That they lived in stormy seas, or were in the habit of diving to considerable depths, is shown by the presence of a ring of bony plates in the sclerotic, protecting the eye from injury or pressure. That they possessed extraordinary powers of vision, especially in the dusk, is certain from the size of the pupil, and from the enormous width of the orbits. That they were carnivorous and predatory in the highest degree is shown by the wide mouth, the long jaws, and the numerous, powerful, and pointed teeth. This is proved, also, by an examination of their petrified droppings, which are known to geologists as "coprolites," and which contain numerous fragments of the scales and bones of the Ganoid fishes which inhabited the same seas.

ORDER VI. SAUROPTERYGIA, Owen (= Plesiosauria, Huxley). This order of extinct reptiles, of which the well-known Plesiosaurus may be taken as the type, is characterised by the following peculiarities:

The body, as far as is known, was naked, and not furnished with any horny or bony exoskeleton. The bodies of the vertebræ

were either flat or only slightly cupped at each end, and the neural arches were anchylosed with the centra, and did not remain distinct during life. The transverse processes of the vertebra were long, and the anterior trunk-ribs had simple, not bifurcate, heads. No sternum or sternal ribs are known to have existed, but there were false abdominal ribs. The neck (fig. 291) in most was greatly elongated, and composed of numerous vertebra. The sacrum was composed of two vertebra. The orbits were of large size, and there