386 ZOOLOGY 



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 conspicuous was this, group 

 that the Mesozoic is called the "Age of Reptiles." All present 

 living reptiles are but a remnant of what was once the greatest 

 and most diversified group of the land vertebrates. 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 in- 

 cluding 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. Usually covered with scales or plates derived from the 

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



2. Digits (3-5) when present provided with claws. 



3. Vertebrae concavo-convex, usually concave in front and 

 convex behind. 



4. Heart three chambered; — that is, auricles completely sepa- 

 rated, but the ventricles only partially so except in the Croco- 

 dilia. 



5. Two aortic arches, a right and a left, in the adult. 



6. GUIs absent in all stages. 



7. Chiefly oviparous ; eggs large, well supplied with yolk, and 

 protected by a leathery shell. 



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 more or less cylindrical 

 body, generally 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 seven or eight feet. The crocodiles 



