| Viperinæ—the Vipers and Crotalidæ. |
| Colubrinæ—the Colubers, Hydridæ, and Boidæ. |
Crocodilia.
Teeth in a single row, implanted in distinct sockets; body depressed, elongated, protected on the back by solid shield; tail longer than the trunk, compressed laterally, and furnished with crests above. The several families are:—
| Crocodilidæ—the Gavials, Mecistops, Crocodiles. |
| Alligatoridæ—Jacares, Alligators, Caiman.[4] |
CHAPTER I.
AMPHIBIA, OR BATRACHIANS.
Those geographers who divide the world into land and sea overlook in their nomenclature the extensive geographical areas which belong permanently to neither section—namely, the vast marshy regions on the margins of lakes, rivers, and ponds, which are alternately deluged with the overflow of the adjacent waters, and parched and withering under the exhalations of a summer heat; regions which could only be inhabited by beings capable of living on land or in water; beings having both gills through which they may breathe in water, and lungs through which they may respire the common air. The first order of reptiles possesses this character, and hence its name of Amphibia, from αμφιβιος, having a double life.
The transition from fishes to reptiles is described by Professor Owen, with that wonderful power of condensation which he possesses, in the following terms:—"All vertebrates during more or less of their developmental life-period float in a liquid of similar specific gravity to themselves. A large proportion, constituting the lowest organised and first developed forms of this province, exist and breathe in water, and are called fishes. Of these a few retain the primitive vermiform condition, and develop no limbs; in the rest they are 'fins' of simple form, moving by one joint upon the body, rarely adapted for any other function than the impulse or guidance of the body through the water. The shape of the body is usually adapted for moving with least resistance through the liquid medium. The surface of the body is either smooth and lubricous or it is smoothly covered with overlapping scales; it is rarely defended by bony plates, or roughened by tubercles. Still more rarely it is armed with spines." Passing over the general economy of fishes we come to the heart. "The heart," he tells us, "consists of one auricle receiving the venous blood, and one ventricle propelling it to the gills or organs submitting that blood in a state of minute subdivisions to the action of aërated water. From the gills the aërated blood is carried over the entire body by vessels, the circulation being aided by the contraction of the surrounding muscles."
The functions of gills are described by the Professor with great minuteness. "The main purpose of the gills of fishes," he says, "being to expose the venous blood in this state of minute subdivision to streams of water, the branchial arteries rapidly divide and sub-divide until they resolve themselves into microscopic capillaries, constituting a network in one plane or layer, supported by an elastic plate, covered by a tesselated and non-ciliated epithelium. This covering and the tunics of the capillaries are so thin as to allow chemical interchange and decomposition to take place between the carbonated blood and the oxygenated water. The requisite extent of the respiratory field of capillaries is gained by various modes of multiplying the surface within a limited space." "Each pair of processes," he adds, "has its flat side turned towards contiguous pairs, and the two processes of each pair stand edgeway to each other, being commonly united for a greater or less extent from their base; hence Cuvier describes each pair as a single bifurcated plate, or 'feuillet.'"
The modification which takes place in the respiratory and other organs in Reptilia, is described in a few words. "Many fishes have a bladder of air between the digestive canal and the kidneys, which in some communicate with an air-duct and the gullet; but its office is chiefly hydrostatic. When on the rise of structure this air-bladder begins to assume the vascular and pharyngeal relations with the form and cellular structure of lungs, the limbs acquire the character of feet: at first thread-like and many jointed, as in the Lepidosiren; then bifurcate, or two-fingered, with the elbow and wrist joints of land animals, as in Amphiuma; next, three-fingered, as in Proteus, or four-fingered, but reduced to the pectoral pair, as in Siren."