ADAPTATIONS 103 



The pelagic crocodiles, for which the name Thalattosuchia 

 has been suggested, are typified by the genus Geosaurus of the 

 upper Jurassic strata, closely allied to which is the much 

 larger Dacosaurus of the English Kimeridge Clay. Geosaurus 

 (" land-lizard ") is an unfortunate title for a pelagic reptile, but 

 less confusion will be caused by its retention than by proposing 

 a more appropriate designation. These crocodiles, which are 

 presumed to have branched off at an early date from the primi- 

 tive terrestrial stock which subsequently gave rise to the 

 modern representatives of the order, discarded the armour of 

 bony plates overlain by horny shields which forms such a dis- 

 tinctive feature of land crocodiles, as being ill-suited to a 

 marine existence, and acquired instead a smooth skin like a 

 whale. In form the body was much elongated, so as to 

 resemble that of a conger-eel, and the long tail probably ter- 

 minated in a fin, while there may have been fin-like expan- 

 sions on the line of the back. The front limbs, which were 

 smaller than the hind pair, were alone modified into perfect 

 paddles, and were probably the chief propellers ; the hind- 

 limbs being apparently held close to the sides of the body. 

 The jaws were long, and the eyes, as already mentioned, fur- 

 nished with a ring of bony plates in the sclerotic, probably for 

 the purpose of resisting the extra pressure of the water when 

 diving. Metriorhynchus of the Oxford and Kimeridge Clays 

 was a nearly allied type. 



Very different in appearance to the pelagic crocodiles were 

 the ichthyosaurs, or fish-lizards (Ichthyopterygia), of which the 

 rude outline of the external form has been preserved in some of 

 the fine-grained clays of the Lias. In these reptiles the general 

 contour was exceedingly fish-like or whale-like : the body being 

 fusiform, and the tail terminating in a large fin, or " flukes," 

 while there was also a back-fin. Unlike that of whales, the tail 

 was, however, vertical, with the backbone continued into the 

 lower lobe. Of the two pairs of paddles, the front ones were 

 much the larger, and doubtless the more important in swim- 

 ming. As in the pelagic crocodiles, the eye was furnished with 

 a ring of sclerotic plates, which may be taken as an indication 

 that some of these reptiles, which attained a length of fully thirty 

 feet, were in the habit of diving to considerable depths in the 

 ocean. Ichthyosaurs present a curious parallelism to whales in 



