450 OSBURN 



in the hand of the Thalottosuchian Geosaiuics and the Cetacean 

 Globiocephahis, in which the extremities of the fingers are at a 

 right angle witli the long axis of the humerus. The opposite 

 condition is found in some of the Ichthyosaurs and Mosasaurs 

 in which the limb is quite straight to the ends of the digits, but 

 in such cases the whole limb is directed backward from the 

 articulation with the girdle sufficiently to compensate for the 

 lack of curvature. It must be noted, however, that in both 

 these groups the most specialized forms show the backward 

 curvature as well as the average of the curved-limb forms (cf. 

 Ichthyosam^us quadriscissus and Baptanodon natans among the 

 Ichthyosaurs and Tylosaurus among the Mosasaurs). Gener- 

 ally speaking, the longer limbs are more curved but this is not 

 without many exceptions. The hand of Gcosmtriis is at once 

 the shortest and most curved that has been noted, while some 

 of the long limbed Ichthyosaurs, e. g., I. aaitirostris, have the 

 limb quite straight. The backward curvature of the limb may 

 be attained in a number of ways in various forms, by one of the 

 following modifications, or, more frequently, by a combination 

 of several of them in the same limb. 



Curved Bones of tlie Limb. — This is exemplified in all the 

 groups where the bones of the limb have not become so short- 

 ened as to obscure any curvature. In the Cetacea it is shown 

 by numerous forms to a greater or less extent. The humerus 

 is generally too short to exhibit any curvature but the radius 

 and ulna are distinctly curved in a number of forms (cf. espe- 

 cially Balcenopterd). The Ichthyosaurian limb is too much 

 shortened to show curvature in any single bone, except that in 

 Mixosaiirus^ the most primitive of the group, the humerus is 

 long enough to show a distinct curve. The longer limbed 

 Plesiosaurs, as a rule, show the curved humerus and femur to 

 a greater or less extent (cf. Plesiosaurus, Chnoliosaiiriis, Pelo- 

 neustes, DolichorJiynchops, etc.). In the Thalattosuchia the 

 humerus is very greatly shortened but the longer femur is very 

 noticeably curved. The Mosasaurs, as a rule, do not exhibit 

 any curvature of the bones, which are greatly shortened, but in 

 Tylosajtnis the humerus is considerably curved. The Pinnipedia 



