2IO 



WATER REPTILES OF THE PAST AND PRESENT 



possibly were too deeply impressed into their structure readily to 

 change, as did those of other sea-reptiles. Some of their remark- 

 able aquatic adaptations have long been known, but only within 

 a dozen years has our knowledge of them become at all complete. 

 Three or four genera have been described, but only a few forms are 

 well known, of which Geosaurus may be taken as most typical. 

 To this we shall confine our descriptions. 



The skull of Geosaurus is rather small in comparison with the 

 length of the body, smaller proportionally than in any living croc- 

 odile, but not much smaller than that of the teleosaurs. The 

 snout is long and slender, much like that of the teleosaurs and 

 gavials, but the bones of the whole upper surface are quite smooth, 

 not roughened and pitted like those of modern forms. The skull 



1 



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Fig. io8. — Geosaurus; skull from side and from above. (After Fraas) 



of Dakosaurus, another genus of thalattosuchians, is much less 

 elongate than that of Geosaurus, but has the other characteristics 

 of Geosaurus. The eyes are provided with a stout ring of sclerotic 

 bones, with a pupillary opening of less than one inch. We have 

 seen that all other strictly aquatic reptiles have similar eye bones, 

 but no other crocodiles have them. The internal openings of the 

 nostrils are large and long, but they are not situated far back, as in 

 the modern crocodiles, not even so far back as in the early teleo- 

 saurs. They had no need of the peculiar breathing apparatus of 

 the amphibious crocodiles, since all their prey must have been 

 water-breathing creatures. Their eyes were directed laterally, 

 not more or less upward, as in their nearest relatives. Nearly all 

 other crocodiles have an opening through the hind end of the lower 



