304 CLASSIFICATION OF VERTEBRATES. 



ards and snakes there are a pair of eversible sacs (hemipenes) 

 opening into the cloaca, and when in repose retracted under 

 the skin of the tail. In chelonians and crocodiles there is but 

 a single penis, formed by a thickened portion of the ventral wall 

 of the cloaca, which is composed of erectile tissue, and can be 

 protruded from the vent. Both types are grooved for transmis- 

 sion of the seminal fluid. The hemipenes of embryo snakes 

 have often been described as rudimentary hind limbs. Hatteria 

 lacks a penis. 



The eggs of reptiles are large and undergo a partial (mero- 

 blastic) segmentation ; the subsequent phases of development 

 being much like that of birds. Most reptiles are oviparous, 

 the eggs being deposited in sand or soil, and left to hatch by 

 the heat of the sun. Some lizards and many snakes, however, 

 are viviparous. 



The following classification of the Reptilia follows most 

 closely that of Lyddeker. The late Professor Cope recognized 

 several more orders, which seem to be but sub-divisions of the 

 theromorpha. 



ORDER I. THEROMORPHA. 



Extinct reptiles, with amphiccelous vertebrae, the notochord 

 frequently persisting intervertebrally ; with a sacrum composed 

 of from two to six vertebrae ; ribs bicipital, their articulation 

 with the vertebrae as in mammals ; quadrate immovable ; teeth 

 in alveoli, and showing much differentiation (occasionally teeth 

 are lacking) ; no sternum ; girdles solid, the pubic and ischiatic 

 bones fused into a continuous os innominatum ; humerus with 

 a foramen (entepicondylar) above the inner condyle. 



The theriomorphs were mostly terrestrial vertebrates, and are especially 

 interesting, since they show features which make many regard them as 

 having been the ancestors of the mammalia. The order appears in the 

 Permian, and dies out in the triassic. 



SUB-ORDER i. PAREIASAURIA (COTYLOSAURIA). 



Teeth homodont, numerous, without diastema ; no temporal fossa ; 

 one occipital condyle ; vertebrae with remains of notochord; two sacral 

 vertebras. Paretasauwts, South African Permian ; Empedias, Permian of 



