REPTILIA. 381 



known as the ainnion and allantois (see 415), the latter of 

 which is important 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. 193). 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. 



406. 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, and protected by a leathery shell. 



8. The embryonic membranes, amnion and allantois first 

 make their appearance in this group. 



