66 WATER REPTILES OF THE PAST AND PRESENT 



With this shortening of the neck and sinuosity of movement 

 there is developed in every case a long trunk as well as a long tail. 

 The trunk becomes more slender and cylindrical, more like that of a 

 snake, with an actual increase of the bones composing it, reaching 

 the great number of forty-three vertebrae in that most sinuous of 

 all water reptiles with legs, Pleurosaurus of the Protorosauria. 

 And the tail, primitively having perhaps sixty or seventy vertebrae, 

 may have as many as one hundred and fifty in the more typical 

 aquatic forms. This elongation of trunk and tail must be of great 

 advantage to the swimming reptile, just as the racing scull is a 

 more perfect type of speedy craft than a flat-bottomed scow. Dr. 

 Woodward has said that the fate of all fishes, if they continue their 

 evolution long enough, is to become eel-like. 



Not only was the tail greatly elongated in swimming reptiles, 

 but it was also more or less flattened. In the beginning of water 

 adaptation the flattening was throughout the tail, as in the living 

 alligators and crocodiles. As the adaptation to water life became 

 more perfect, the flattening became more and more restricted to the 

 extremity; that is, the flattening begins like that of a salamander 

 and in the end becomes like that of a fish, a terminal fin. And 

 some of the actual stages in the evolution of the fish-like fin have 

 been observed by Dr. Merriam in the earlier and more primitive 

 ichthyosaurs of California. In those animals swimming chiefly 

 in a horizontal direction the tail fin has become like that of fishes, 

 that is, vertical; but in those animals which use the tail chiefly for 

 ascending and descending rapidly in the water the fin is developed 

 in a horizontal position, examples of which are seen in the flukes of 

 whales and sirenians. 



All animals living upon the land require firm articulations 

 between the different bones of the skeleton, and especially between 

 the vertebrae, for the support and control of the body. Among 

 aquatic animals there is a strong tendency toward looseness 

 of joints, with increasing flexibility. Fishes have the articular 

 processes between the arches of the vertebrae feebly or not at all 

 developed, and the centra or bodies of the vertebrae have thick 

 pads of cartilage between them. Firm union between the verte- 

 brae would restrict freedom of movement, and firmness is not 



