AND AVES OF NORTH AMERICA. 27 



Opisthotic united with exoccipital ; brain case not closed before prootics ; palatines 

 united all round ; sacrum of two or one vertebra ; pubes transverse. 



LACERTILIA. 



Opisthotic distinct and distally free from the cranium ; brain case partly closed before 

 prootic, palatines united. 



PYTHOXOMORPHA. 



Opisthotic distinct, free from cranium except proximally ; brain case nearly closed 

 anteriorly, palatines attached behind only. 



OPHIDIA. 

 ICHTHYOPTERYGIA. 



Elements of the limbs beyond the humerus not differentiated, in indefinite number. 



Postorbital and supertympanic bones over the temporal fossa. 



Quadratum solidly united by suture with the prootic opisthotic and quadra tojugal. 



Sacrum none. 



Pubes and ischia transverse and in contact. Neural arches free. 



Premaxillary divided. 



ARCHOSAURIA. 



Elements of the lirnbs, of the pes and manus differentiated, in definite number. 



" Postorbitals and supra temporals " of Owen wanting. 



Quadratum immoveably united by suture with opisthotic, prootic and quadratojugal. 



Sacrum of from one to six vertebrae. 



Neural arches attached by suture in most. 



Premaxillary divided. 



Cranial walls cartilaginous anteriorly. 



Palatine bones in contact with maxilliaries, and united by suture with them. 



Circulatory System (known only in the Crocodilia). Heart with complete septum of the 



ventricles ; a communication between aorta roots. 

 Nervous System (known only in the Crocodilia). Cerebellum with small lateral lobes and 



weak plicae. 



TESTUDINATA. 



Parts of limbs differentiated. 



Dorsal vertebrae without mobility : no clavicle, a procoracoid continuous with scapula : 



ilium vertical, acetabulum complete. 

 Sternum not in connection with coracoid, composed at least of eight or more bilateral 



elements, and extending posteriorly to beneath the pelvis. 



