ICHTHYOSAURI A 119 



increased in number and so changed in form and relations that 

 they bear little resemblance to the corresponding bones of other 

 reptiles. They are merely polygonal platelets of bone, articulating 

 on all sides and fitting closely together, pcrniitting flexibility, but 

 not much else. 



It is now believed that the increase, not onl\' of additional 

 digits, sometimes to as many as ten in each hand and foot, but of 

 the finger and toe bones as well, was the result of a sort of vegetative 

 reproduction. The margins and ends of the flippers were doubtless 

 hardened by cartilage or fibrous material, and because of the action 

 of the limbs this cartilagenous material broke up into nodules each 

 of which took on ossification finally. Among the whales, where 

 hyperphalangy also occurs, though to a less extent, it has been 

 thought that the increase in number has been due simply to the 

 ossification of the parts of each bone normally present, that is, to 

 the epiphyses, which became separated from the shaft of each bone. 

 But this explanation will hardly suffice for the fingers and toes of 

 the plesiosaurs and ichthyosaurs, for there are altogether too many 

 such ossifications; and besides, the bones in these animals, as in 

 most reptiles, did not have epiphyses, or terminal separate ossifica- 

 tions of the bones of the skeleton. 



It will be observed from the figures that the arm and thigh 

 bones of Ichthyosaurus are very much shortened — a striking adapta- 

 tion to water life, so conspicuously seen in the modern whales 

 and dolphins as well as in the mosasaurs, thalattosaurs, etc. So 

 characteristic indeed is this shortening that, were every other bone 

 of the skeleton of an ichthyosaur unknown save the humerus or 

 femur, it would be quite certain from these alone that the animal 

 was thoroughly aquatic in habit. 



About sixty years ago a rather aberrant form of ichthyosaur. 

 now known as Mixosaurus, was discovered in rocks of Triassic age. 

 that is, of much greater age than any ichthyosaurs previously found, 

 in which not only the forearm but also the lower leg bones were 

 longer, resembling more the corresponding bones of land animals. 

 It was from the examination of specimens in 1887 of these mixo- 

 saurs that the late Professor Baur became convinced that the 

 ichthyosaurs were the descendants of land reptiles, and not directly 



