Ii6 WATER REPTILES OF THE PAST AND PRESENT 



animals were in the external form — so fish-like that the name Ich- 

 thyosaurus is not misleading, though Koenig gave it in the mistaken 

 belief that they were really allied to the fishes. When to these 

 external features certain other fish-like details of the skeleton are 

 added, we do not wonder that the early observers were so long in 

 doubt about them. A more careful examination of the skeleton 

 will, however, disclose so many truly reptilian characters that their 

 external appearance and habits lose all significance. 



The vertebrae are deeply biconcave and fish- like, it is true, but 

 a consideration of the reasons therefor will convince us that any 

 other kind of vertebrae would be more remarkable. At the time 

 when the ichthyosaurs must have originated, at the time when the 

 first known ichthyosaurs appeared in geological history indeed, all 

 reptiles had biconcave vertebrae, and for the most part at least 

 deeply biconcave ones. The vertebrae remained fish-like through- 

 out all their history, perpetuating their type until most other rep- 

 tiles had developed a firmer one, because such vertebrae were best 

 adapted for the quick, pliant movements of the spinal column so 

 necessary for the well-being of the animals in the water. In the 

 modern dolphins, animals in shape, size, and habits most wonder- 

 fully allied to what these old reptiles must have been, the small, flat- 

 ended vertebrae are widely separated by disks of flexible cartilage. 



Not only were their vertebrae fish-like in form, but there are 

 other characters in the spinal column of a primitive or generalized 

 nature. As in all aquatic animals, the articulating processes 

 between the vertebrae are either weak or wanting in the posterior 

 part of the column. And they were not only small, but were situ- 

 ated, in many, high up, very remarkably resembling the peculiar 

 arrangement of the articulations in the dolphins. 



There is no sacrum, that is, there were no united vertebrae pos- 

 teriorly for the attachment and support of the pelvis, as no such 

 support was needed. In only one other group of aquatic reptiles 

 was the sacrum lost, though it has wholly disappeared in the ceta- 

 ceans and sirenians among mammals. The chevron bones of the 

 tail, usually bony arches on the under side of the tail for the pro- 

 tection of the blood-vessels, in crawling reptiles, were very imper- 

 fectly developed in the later forms, though normal in shape in the 

 early ones. The ribs are numerous, long, and slender, very much 



