216 EEPTILIA. 



The femur is short compared with the size of the animal, the tibia 

 still shorter, and the fibula extremely slender. The astragalus is fused 

 with the distal end of the tibia. There seem to have been three hoofed 

 digits in the hind foot. The dermal armour comprises numerous bony 

 tubercles and bosses, of which the arrangement is not definitely known. 

 Triceratops prorsus, restored in the accompanying fig. 132, attained a 

 length of at least eight metres. The known remains include one nearly 

 complete skeleton from the uppermost Cretaceous (Laramie Formation) 

 of Wyoming, U.S.A. 



In Sterrholophus, of which the skull is shown of one- 

 twentieth the natural size in fig. 184, the whole of the posterior 

 face of the parietal crest must have been covered with the 

 ligaments and muscles supporting the head ; whereas in 

 Ceratops and Triceratops a wide margin of this face was clearly 

 exposed and protected by a thick horny covering. 



ORDER 8. CROCODILIA. 



It is as yet impossible to distinguish the Triassic ancestors 

 of the Crocodilia from those of the Rhynchocephalia and 

 Dinosauria. True crocodiles, however, are known to range 

 from the earliest Jurassic rocks upwards. In all these forms, 

 the teeth are confined to the margin of the jaws, there is a 

 great development of the secondary palate, and there is a 

 tendency to the enclosure of the eustachian passages by bone. 

 As in modern crocodiles, also, the pubis is always excluded 

 from the acetabulum and carried on an anterior process of the 

 ischium. Some marked changes, however, have taken place 

 iii the course of their evolution. In all the known Jurassic 

 crocodiles the hinder margin of the secondary palate is formed 

 by the palatines, there being no outgrowths from the ptery- 

 goids; while the lateral eustachian passages occupy merely 

 open grooves on the basisphenoid bone. All except very few 

 of the latest forms possess amphicoelous vertebrae ; and it is 

 only in the Upper Jurassic that short- and broad-headed genera, 

 with small supratemporal vacuities, begin to appear. In the 

 Cretaceous period all the known crocodiles, except two remark- 

 able genera from northern Patagonia, exhibit the extended 



