KEPTILIA. 187 



CLASS G. REPTILIA. (THE REPTILES.) 



THE 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 (CROCO- 

 DILIA) is represented by two species (Alligator mississippiensis 

 Daudin, and the rare Crocodilus americanus Seba,) in the lowlands 

 of the South. 



Orders of Keptilia. 



a. Body covered with imbricated scales ; vent a cross-slit ; bones of skull 



separate; jaws with teeth; dorsal vertebrae and ribs movable. 

 b. 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 j no tympanum. 



OPHIDIA, XXVIII. 



bb. 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 LACERTJLIA, XXIX. 



act. 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. 



ORDER 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 



