STRUCTURE OF THE VERTEBRATES 57 



Sufficient identification of the class Reptilia is: (1) the cover- 

 ing of ectodermal scales; (2) large, yolk laden eggs; (3) a vari- 

 able temperature, depending to a large extent on the environment ; 

 and (4) the allantois and amnion. Other differences will become 

 clear in later discussions. 



For a study of mammal evolution, only the first three of the 

 following orders are necessary. The others will assist in under- 

 standing the general scheme of vertebrate evolution. The first 

 order is usually studied in anatomy courses as representative of 

 the earliest reptiles. The other two are extinct ancestral stocks 

 from which the mammals evolved. 



Order Chelonia. The turtles, tortoises and terrapins are the 

 most primitive, and at the same time the most superficially spe- 

 cialized of living reptiles. They are easily identified by their 

 bony covering, forming a dorsal and a ventral ''shell". This is 

 composed of dermal plates which become fused with the ribs, and 

 serves as a protection to otherwise inefficient, lethargic animals. 



The skull shows relationship with the earliest reptiles, and this 

 is corroborated by the simple development of their vascular and 

 nervous systems. The group, in their present form, is very an- 

 cient; and during their history they have become adapted to al- 

 most every environment. Some inhabit dry, sandy regions, their 

 burrows giving them the popular name of "gophers"; others, like 

 the Box Tortoise, are found in more or less moist regions; the 

 small pond turtles and the terrapins are largely aquatic; and 

 the Sea Turtle is so highly modified for life in the ocean that it 

 rarely, or never, leaves the water except at breeding time. In 

 this the reptiles reverse the breeding habits of the amphibia. No 

 matter how specialized a turtle is for aquatic life, it always goes 

 to the land to lay its eggs. The degree of specialization for water 

 life can, within limits, be determined by the cross section of its 

 shell: the deeper the shell in proportion to its width, the greater 

 its adaptation for land; and conversely, the flatter the shell, the 

 more modified for water. 



In size the turtles vary through equal limits. The largest living 

 specimen is the Galapagos tortoise which is found only on the 

 barren islands off the coast of South America. But even this huge 

 chelonian is dwarfed by some of the fossil specimens which have 

 been found in India and other regions. 



