448 OSBURN 



some of the long-tailed reptiles like the Mosasaurs may have 

 used the tail for propulsion from the very beginning of their 

 aquatic existence, as the newts and certain semi-aquatic lizards 

 are known to do. The tail is a much more efficient organ of 

 propulsion than the limbs, however, and in forms where the 

 anatomical difficulties in the way are not too great a tail fin is 

 developed which relieves the limbs of the greater part of their 

 work and leaves them with only the function of equilibration to 

 perform. This results in the reduction in size of the fore limbs 

 and the degeneration or loss of the hind limbs in the most ad- 

 vanced forms. The final result, then, is as near an approach as 

 possible to fish-like conditions of locomotion, with an expanded 

 tail fin for a propeller and thin, flexible paddles on the sides for 

 balancing. 



Naturally, the forms which show the greatest adaptation to 

 aquatic life are those such as the Cetacea and Ichthyosauria 

 which have entirely given up their land relations, while those 

 which come upon the land occasionally, as the Chelonia and 

 Pinnipedia at the breeding season, retain of necessity certain 

 characters of the terrestrial type of limb. In the present paper 

 the groups to be considered are those whose adaptation has 

 progressed to the extent of modifying the limb skeleton, especi- 

 ally the Cetacea, Sirenia, Ichthosauria, Plesiosauria, Mosasauria 

 and Thalattosuchia, and the less modified Pinnipedia and 

 Chelonia. 



In the limbs of these forms the following modifications seem 

 to be the most usual and of the greatest importance in the 

 change from the terrestrial to the natatory type of limb : 



1. Abbreviation of the limb" as a whole. 



2. Curvature or backward extension of the limb. 



3. Distal dilatation of the limb. 



4. Parallelism or convergence of the fore and hind limbs. 



5. Tendency toward loss of hind limbs. 



6. Tendency toward similarity in shape and function of all 

 the bones of the limb. 



7. Elongation of digits. 



8. Hyperphalangy, or increase beyond the normal number of 

 phalanges. 



