REPTILIA. DINOSAURIA. 205 



vary greatly in size and in the structure of the limbs, some 

 approach close to the type of structure met with in birds, 

 others are allied to crocodiles, 



Passing to the more detailed characters : there is some- 

 times a well-developed exoskeleton having the form of bony 

 plates or spines. The vertebrae may be solid or their centra 

 may be hollowed internally ; their surfaces may be flat, bi- 

 concave or opisthocoelous. The sacrum is composed of from 

 two to six vertebrae. 



As regards the skull, the quadrate is large and fixed, and 

 supratemporal and infratemporal fossae bounded by bone occur. 

 The teeth are more or less laterally compressed, and often 

 have serrated edges ; they may be placed in distinct sockets 

 or in a continuous groove. The ribs have capitula and tuber- 

 cula, and sternal ribs often occur. The scapula is very large, 

 the coracoid small, and there is no precoracoid, or T-shaped 

 interclavicle. Clavicles are only known in a few cases. In 

 the pelvis the ilium is elongated both in front of, and behind, 

 the acetabulum, sometimes the pre-pubis, sometimes the post- 

 pubis is the better developed. The anterior limbs are shorter 

 than the posterior, and the long bones are sometimes solid, 

 sometimes hollow. 



There are three well-marked suborders of the Dinosauria. 



Suborder (1). SAUROPODA 1 . 



The reptiles belonging to this group were probably quadru- 

 pedal and herbivorous. 



They have the cervical and anterior trunk vertebrae 

 opisthocoelous, while the posterior vertebrae are biconcave; 

 all the presacral, and sometimes the sacral vertebrae are 

 hollowed internally. The teeth are spatulate and without 

 serrated edges, they are always planted in distinct sockets, 

 and some of them are borne by the premaxillae. 



1 The diagnostic characters of the different groups of Dinosaurs are 

 in the main those given by von Zittel. 



