260 KANSAS UNIVERSITY SCIENCE BULLETIN. 



I believe, the result of adaptive changes from terrestrial to 

 aquatic life : 



Elongation of the head, with attenuation of the facial region. 



Elongation of trunk and tail, but especially of the latter, with 

 progressive weakness of the zygapophysial articulations pos- 

 teriorly. 



Shortening of the neck. 



The acquirement of a caudal fin. 



The presence of sclerotic plates. 



Recession of the external nares. 



Absence of sacrum and the obsolescence or absence of the 

 sternum. 



Greater slenderness and small size or loss of the hind limbs. 



Broadening and shortening of the limb bones. 



Hyperphalangism and hyperdactylism. 



Smoothness of the skin. 



Sponginess of the bones of the skeleton, with a larger pro- 

 portion of organic matter. 



Increase in number and decrease in size or loss of the teeth. 



In the fish-eating swimming vertebrate, speed and the ability 

 to quickly change the direction of movement must be essential 

 for the welfare of the individual. These have been obtained 

 by such changes in the form of the body as decrease resistance 

 to the water, and by the development of organs to give strong 

 propulsion and rapid evolution. Propelling power may be fur- 

 nished by large and strong paddles or flippers, but more effect- 

 ive control and power are given by a long and flexible tail with 

 a fin-like termination. In reality, it is found that, as the tail is 

 large and powerful, the limbs, especially the hind ones, lose 

 their propelling power and become organs of equilibration. 

 A large tail and large paddles are not found together. When, as 

 is sometimes the case, the tail is small, the hind limbs take on 

 more the character of guiding or sculling organs than of oar- 

 like paddles, as is probably the case in many pinnipeds. 



The elongation of the head and attenuation of the facial por- 

 tion are best exemplified by the ichthyosaurs, some plesiosaurs, 

 the semiaquatic gavial, and the more aquatic geosaurs, and 

 perhaps, also, by some of the rhyncocephalians — all, probably, 

 with like food habits. Among some of the odontoceti only is 

 there an approach to the reptilian slenderness of the face ; an 



