Chap, in.] REPTILES : BIRDS. 97 



the dermis are definitely arranged, and form a 

 " carapace," which is generally, though not always 

 (Trionyx), covered by horny epidermic plates, which 

 form the " tortoise-shell." They exhibit a primitive 

 character in the retention of the five digits in either 

 limb, but diverge from the typical organisation in the 

 loss of teeth ; an interesting series of modifications, in 

 relation to their mode of life, are exhibited by the 

 limbs. .In the tortoises, which are terrestrial, the 

 digits are free ; in the amphibian terrapenes there is a 

 partial web, which is more complete in the Triony- 

 chidse ; while the marine Cheloniidse have the digits 

 completely covered by skin, so that they form 

 flattened swimming fins. 



4. Crocodilia, or crocodiles and alligators, are 

 the only reptiles in which the heart is four-chambered ; 

 like the Chelonia, they have the quadrate immovably 

 connected with the side of the skull, the penis is 

 simple and solid, and the anal orifice is rounded. The 

 teeth are set in distinct sockets, and are never found 

 on any bones but the maxillae, premaxillae, and 

 dentaries. They have returned to an amphibious or 

 aquatic mode of life, in correlation with which their 

 feet are webbed, the nostrils can be closed, and the 

 tympanic membrane of the ear covered over. 



B. Aves, or birds, are Sauroids with feathers, 

 with never more than three digits in the manus, or 

 four in the pes ; three of the metatarsals are ankylosed 

 with one another, and with the distal tarsal bone. 

 The blood is hot, and there is only a single systemic 

 aorta. All recent forms are toothless. Physiologically, 

 if not also morphologically, the recent forms are 

 divisible into : 



I. Ratitae, in which the ventral surface of the 

 sternum is broad and flattened, and the fore-limb does 

 not form a functional wing ; such are the ostrich and 

 the cassowary. 

 H 16 



