REPTILIA 385 



the amnion and allantois (see 423), the latter of which is im- 

 portant in embryonic respiration, that is, before hatching or 

 birth. The group reached its culmination in numbers, variety 

 and size in the Mesozoic age. So true is this that the Mesozoic 

 is called the "Age of Reptiles." Those we have at present are 

 to be looked upon as specialized and, in some instances (snakes) 

 perhaps, degenerate remnants of the first vertebrate class wholly 

 to give up breathing by means of gills. In the Mesozoic era 

 there were immense swimming, fish-like forms (ichthyosaurs and 

 plesiosaurs) which ruled the seas; powerful terrestrial dinosaurs, 

 often walking on their hind legs, and including the largest land 

 animals known to have lived; and others, with membranous 

 wings like the bat, the first vertebrates to learn the art of flying 

 (Fig. 196). With the exception of a few marine turtles, the 

 boas and pythons, and the alligators and crocodiles, the living 

 species are for the most part small animals. 



414. General Characteristics. 



1. Reptilia are usually covered with scales or plates derived 

 from the dermis (bony), or the epidermis (horny), or from both. 



2. The (3-5) digits when present are provided with claws. 



3. The vertebrae are concavo-convex, usually concave in 

 front and convex behind. 



4. The heart is three chambered; that is, the auricles are 

 completely separated, but the ventricles are only partially so 

 except in the Crocodilia. 



5. There are two aortic arches, a right and a left, in the adult. 



6. Gills do not occur at any period. 



7. Reptiles are chiefly oviparous; the eggs are large, well 

 supplied with yolk, arid protected by a leathery shell. 



8. The embryonic membranes, amnion and allantois first 

 make their appearance in this group. 



415. The Reptiles are very diverse in form. Perhaps the 

 lizards may be taken as typical, with cylindrical body, more or 

 less distinct head and neck, distinct tail, and usually two pairs 

 of appendages, each possessing five digits armed with claws. 

 They are mostly small animals, though one species is known to 

 attain a length of five feet. The crocodiles and alligators are 



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