REPTILIA. 187 
Crass G.— REPTILIA. (THE REPTILES.) 
TuE Reptiles are cold-blooded air-breathing vertebrates, usually 
scaly or covered with bony plates, never with feathers or hair, 
The limbs when present are usually adapted for walking, sometimes 
for swimming. There is an incomplete double circulation of the 
blood; the septum between the two ventricles being usually want- 
ing or imperfect. There is no metamorphosis after leaving the 
egg, and the eggs are large and mostly provided with a leathery 
skin. The skeleton is usually firm, and the nervous system is 
better developed than in the preceding groups. There are various 
other anatomical and embryological peculiarities of the Reptiles, 
too numerous to be noticed here. We may say however that the 
Reptiles are obviously distinguished from the Birds by the absence 
of feathers, and from the Batrachians by the presence of scales, 
and by the absence of gills after leaving the egg. The extinct 
forms of Reptiles are numerous, and their close relation with the 
earlier birds show the propriety of uniting the two classes in a 
single group, Sauropsida. The three orders represented in our 
fauna are well distinguished from each other. A fourth (CRroco- 
DILIA) is represented by two species (Alligator mississippiensis 
Daudin, and the rare Crocodilus americanus Seba,) in the lowlands 
of the South. 
Orders of Reptilia. 
u. Body covered with imbricated scales ; vent a cross-slit ; bones of skull 
separate; jaws with teeth; dorsal vertebrae and ribs movable. 
6. Mouth very dilatable; bones of mandible (and of head generally) united 
by ligaments; limbs wanting or represented by short spurs on sides of 
vent; no shoulder girdle ; no eyelids; no tympanum. 
Opnipia, XXVIII. 
6b. Mouth not dilatable; bones of mandible united by a bony suture in 
front; limbs 4 (rarely obsolete); shoulder girdle present; eyelids and 
: tympanum usually evident. . . . . . ». Lacerrmia, XXIX. 
aa. Body short, depressed, enclosed between two bony or cartilaginous shields 
(carapace; plastron), from which the head, limbs, and tail may be pro- 
truded; jaws with a horny shield and no teeth; vent roundish or longi- 
tudinal, plaited.. . . . . « « « « » + TESTUDINATA, XXX, 
OrpeR XXVIII. OPHIDIA. (THE Serpents.) 
Reptiles with elongate, terete bodies, obsolete limbs, and with an 
epidermal covering of imbricated scales, which is shed as a whole 
and replaced at regular intervals; the mouth very dilatable; the 
