CHAPTER XII 

 REPTILIA 



Reptilia are cold-blooded vertebrates with an epidermal skeleton of 

 scales, and often with an armor of bony dermal plates. They never 

 breathe by gills and do not have the tadpole stage seen in the Amphibia. 

 Many of them lay eggs of a large relative size, inclosed in a leathery or 

 a calcareous shell. The centra of the vertebrae have spheroidal articu- 

 lar surfaces. There is one occipital condyle and the mandibles 

 usually, and other bones frequently, bear teeth. The ventricles of the 

 heart are usually incompletely separated and there are always two 

 aortic arches in the adult. 



The reptiles, though clearly distinguished from them by having 

 cold instead of warm blood and scales instead of feathers, are closely 

 related to the next class, the birds, with whom they are frequently 

 grouped under the name Sauropsida. In the same way the reptiles, 

 birds and mammals, since they all possess the embryonic structure 

 known as the amnion, are ground together under the name, Amniota. 



During the Mesozoic period the. rep tiles reached their maximum, 

 both as to numbers and individual size, so that this period is often 

 spoken of as the "age of reptiles;" the largest of the living reptiles are 

 pygmies compared to the huge individuals of this age. 



The reptilia are variously classified by different authors, being di- 

 vided into from nine to fifteen or more orders. Of these orders, four 

 include the 4000 or more species of living forms, which, of course, 

 are the only ones of economic importance except as mere specimens, 

 or as determiners of geological horizons. 



The four orders of living reptilia are as follows: 



i. Chelonia or Testudinata. These forms are easily recognized by 

 the bony or leathery carapace covering the dorsal side of the body and 

 a flat plastron on the ventral side; these bones are usually covered with 

 horny epidermal plates; the appendages are clawed and adapted for 

 use on land, or are paddle-like for use in swimming. The jaws are 

 armed with sharp bony edges instead of teeth; tympanic membranes 



