ADAPTIVE MODIFICATIONS OF LIMB SKELETON 449 



9. Hyperdactyly, or increase beyond the normal number of 

 digits. 



10. Formation of smaller skeletal parts. 



11. Concentration of all parts except the digits. 



12. Cartilaginous progression in joints. 



13. Loss of movable articulations in the limb. 



14. Loss of tuberosities for muscle attachment. 



Bearing in mind now the fact that the various forms have 

 been derived from different sources, and the possibility that in 

 the process of their adaptation to aquatic life they may not all 

 have followed exactly the same course because of special en- 

 vironments or of innate differences in potentiality for modifica- 

 tion, let us proceed to examine the above points as they are 

 represented in the various groups of animals. 



Abbreviation of the Limb. 



In all the forms of animals under consideration that have 

 progressed far enough in aquatic adaptation to lose their land 

 relation there is noticeable a very great shortening up of the 

 limb as a whole. So far is this process carried in the Cetacea, 

 Ichthyosauria and Mosasauria that the total length of the limb 

 in many cases may be greatly exceeded by the diameter of the 

 body. Especially is the abbreviation marked in those forms 

 which have the tail fin well developed, as the Cetacea and 

 Sirenia in which the hind limb is lost entirely, and in Ichthy- 

 osaurs some of which have the hind limb very much reduced. 

 The shortening always begins in the proximal end of the limb, 

 with the propodial and epipodial elements, and the digits are 

 not shortened until actual degeneration sets in. This question 

 will be given further mention in certain of the following topics. 



Curvature or Backward Extension of the Limbs. 

 Almost without exception the natatory limb is curved back- 

 ward. This may be seen in the Cetacea and Pinnipedia among 

 the Mammals and in the Plesiosauria and Thalottosuchia and 

 most Ichthyosaurs and Mosasaurs as well as in the marine 

 Chelonia, among Reptiles. The extreme of curvature is found 



