VERTEBRATA. 



197 



Pipidce. 

 Pipa. 



Dactylethra. 

 Myobatrachus. 



EanidcB (Frogs). 

 Pseudis. 

 Eana. 



Ceratophrys. 

 Pleurodema. 

 Cystignathus. 

 Limnodynastes. 

 Discoglossus. 

 Alytes. 

 Asterophrys. 



Class III. REPTILIA. (Eeptiles.) 



Vertebrate animals, breathing by lungs throughout life ; the 

 body covered with bony plates or with scales. Blood cold. 

 With or without limbs. An anmion and allantois. 



The heart has two auricles and a ventricle, but in crocodiles 

 the latter is divided by a septum. The heart has two aortic 

 arches. The lower jaw is attached to the skull by the interven- 

 tion of an os quadratum, as in birds, and there is only a single 

 occipital condyle on each side. The teeth, which are sometimes 

 absent, are conical, and not adapted for crushing or tearing ; the 

 food is consequently swallowed entire. The teeth are not pro- 

 vided with fangs at the root, and they are generally anchylosed 

 to the jaw. The ribs are always well developed, but the sternum 

 is often wanting. The cavity of the thorax is not separated from 

 the abdomen by a diaphragm. The segmental structure of the 

 lateral muscles is still shown in the crocodile, but it is almost lost 

 in the Ophidia and Chelonia. 



A " purposive weapon " is found in the embryo of snakes and 

 lizards, a sharp tooth being developed in the premaxillary bone, 

 wherewith they cut through the egg-shell ; it disappears in the 

 adult. 



There are about 1900 living species in this class ; of the extinct 

 forms over 400 are known, mostly belonging to the Secondary 

 period. There are ten orders ; Ichthyopterygia and Sauropterygia 

 form the Enaliosauria of some writers. 



