CHAPTER XXIII 



SUB-PHYLUM IV. CBANIATA 



DIVISION II. GNATHOSTOMATA 



SUB-DIVISION II. AMNIOTA 



CLASS III. REPTILIA 



THE name Reptile denotes literally anything that crawls (Lat. 

 repo or repto, I crawl). Zoologically the term denotes the lowest 

 Amniota, which are cold-blooded, and whose eggs are large and 

 provided with plenty of yolk. 



Perhaps the most characteristic feature of Reptiles is the 

 nature of their skin. They are typically covered with scales which 

 are widely different from the scales of fish. The latter are essen- 

 tially areas of the dermis hardened by the deposition of lime with 

 sometimes the addition of a layer of crystals from the basal ends of 

 the ectoderm cells (enamel). 



The scale of the Reptile on the contrary is nothing but an area 

 of the horny layer of the skin where the cells are converted into 

 horn or keratin and are adherent to one another. In the mass of 

 the scale the horn is rendered brown by the presence of pigment, 

 but the outermost layer is composed of clear cells and is known as 

 the epitrichial layer. A corresponding layer covers the embryos 

 of Birds and Mammals, but is shed before birth. A sloughing 

 or ecdysis of the scaly epidermis is a constant feature of the 

 Reptilia. It may take place bit by bit, or as is the case with 

 many Sauria the whole "skin" is cast in one piece (Fig. 279). 



In very many living Reptiles, and in a great many extinct ones, 

 there are numerous small dermal bones embedded in the skin termed 

 osteoderms. These do not belong to the same series of membrane 

 bones as those which give rise to the dermal bones of the skull or 

 the scales of fish. They are to be looked upon as later and secondary 

 ossifications of the dermis arising long after the primary series had 

 become separated from the skin and sunk inwards. In many 



