REPTILIA. THEROMORPHA. 191 



or formed of cartilage bone, but never of membrane bone ; 

 it differs from that of birds also in the fact that it does not 

 ossify from two or more centres. An interclavicle is generally 

 present. There are always more than three digits in the 

 manus, and never less than three in the pes. In all living 

 reptiles the ilia are prolonged further behind the acetabula 

 than in front of them, and the bones of the pelvis remain as 

 a rule, distinct from one another throughout life. 



The pubes (pre-pubes) and ischia both commonly meet in 

 ventral symphysis, and the acetabula are wholly or almost 

 wholly ossified. The metatarsals are not ankylosed together. 



Order 1. THEROMORPHA '. 



This order includes a number of mainly terrestrial, extinct 

 reptiles, which differ much from one another, and show remark- 

 able points of affinit} r on the one hand with the Labyrintho- 

 dont Amphibia, and on the other with the Mammalia. The 

 vertebrae are nearly always amphicoelous and sometimes have 

 notochordal centra. The skull is short and has the quadrate 

 immovably fixed. There is an interparietal foramen, and gene- 

 rally large supratemporal fossae bounded by supratemporal 2 

 arcades, but with no infratemporal 2 arcades; Elginia however 

 has the whole of the temporal region completely roofed over. 



The teeth are placed in distinct sockets and are very 

 variable in form, the dentition sometimes resembling the 

 heterodont dentition of mammals. The humerus has distinct 

 condyles and an ent-epicondylar foramen 3 as in many mammals. 



The pubis is fused with the ischium, and both pectoral and 

 pelvic girdles are remarkably solid. The obturator foramen 



1 T. H. Huxley, Quart. J. Geol Soc., vol. xv. p. 649, 1859. R. Owen, 

 Catalogue of Fossil Reptiles of S. Africa in the British Museum, London, 

 1876. H. G. Seeley, various papers published in the P. R. Soc. London, 

 and Phil. Trans. 



2 See pp. 281283. 



3 An ent-epicondylar foramen is one piercing the humerus on its 

 inner side just above the condyle. 



