THh hXTlNC'I DINOSAURS 2389 



on the lower surface of the body form two distinct shields. It is remarkable that 

 among these extinct crocodiles some are met with having broad and short snouts 

 like the modern alligators, while others have long and narrow snouts like the ga- 

 rials. In the Wealden and Purbeck rocks, underlying the Chalk, some of these 

 crocodiles, such as the short-snouted Swanage crocodile (Goniopholis} , resembled 

 living types in having the socket of the eye communicating freely with the lower 

 temporal fossa, although they were distinguished by the plates of the back articu- 

 lating by means of a peg-and-socket arrangement. In still older formations, 

 such as the Lower Oolites and L,ias, there were, however, many long-snouted 

 crocodiles, such as the steneosaurs (Steneosaitrus} and pelagosaurs (Pelagosaurus} , 

 in which the socket of the eye is divided from the lower temporal fossa by a bony 

 bar, as shown in the figure on p. 2369. Moreover, in these forms the upper tem- 

 poral fossa (7" in the figure cited) was larger than the socket of the eye, whereas in 

 all living forms the former is much the smaller of the two, and may even be obliter- 

 ated. Another group of crocodiles, the metriorhynchs (Metriorhynchus] , of 

 the Oxford and Kimeridge Clays, were remarkable in having no bony armor at 

 all, in which respect they were more specialized than any of their living cousins. 

 In general, however, the earlier extinct crocodiles, as will be gathered from the 

 foregoing remarks, were decidedly of a less specialized type than those of the 

 present day; and as a gradual transition can be traced in these respects from the 

 oldest to the most recent, the group affords a very interesting instance of pro- 

 gressive evolution. In the very oldest of the secondary rocks, namely, the Trias, 

 there occur, both in Europe and India, certain very remarkable long-snouted rep- 

 tiles, known as Parasuchians, which appear in some respects intermediate between 

 crocodiles and tuateras. Thus, while they resembled the former in the nature of 

 their teeth, bony armor, ribs, and vertebrae, they approximated to the latter in 

 the structure of the skull, abdominal ribs, and probably of the collar bones and 

 interclavicle. 



THE EXTINCT DINOSAURS 

 Order DINOSAURIA 



Nearly allied to crocodiles are those remarkable extinct reptiles from the rocks 

 of the Secondary period, which include among their number the most gigantic of 

 all land animals, and likewise those members of the reptilian class which make the 

 nearest approximation in their organization to birds. During that epoch of the 

 earth's history in which the Chalk and underlying Oolitic rocks were deposited, 

 when mammals were represented by a few small forms of lowly type, these strange 

 reptiles were the dominant animals on land; some progressing in the ordinary 

 lizard-like manner, while others stalked on their hind-limbs like birds. To give 

 some idea of the enormous dimensions attained by some of these creatures, it may 

 be mentioned that the thigh bone of one species measures sixty-four inches, while 

 the total length of its skeleton is estimated to have been between .sixty and eighty 



