ADAPTIVE MODIFICATIONS OF LIMB SKELETON 451 



and marine Chelonia still retain their land connection to some 

 extent so their limbs are less modified than in the other groups, 

 but even here the radius and ulna and corresponding elements 

 of the hind Hmbs are in some forms very distinctly curved. 

 The other bones of the limbs are naturally too short to show 

 any curving, except very occasionally, e. g., the first metacarpal 

 of Geosaiiriis. 



In some cases where the humerus and femur are not notice- 

 ably curved the anterior border is longer than the posterior and 

 as a result the distal articular surface is set at an angle to the 

 long axis of the bone. The result is the same as that produced 

 by the curving of the bones. Examples of this angulation of 

 the joint may be seen in all the groups, but it is by no means 

 constant and only about half of the forms examined show it, the 

 others being indifferent or actually angulated in the reverse di- 

 rection. However, any forward angulation of the joint is always 

 more than compensated for by some other means, usually by 

 having the radius longer than the ulna, etc. (cf Cimoliosaiiriis 

 euryinenis or Mixosauriis). 



The Greater LengtJi of the Radius and Tibia as Compared zvith 

 the Ulna and Fibula. — This condition results in a backward 

 angulation at the distal extremities of these bones very much 

 as in the longer anterior border in the humerus or femur. It 

 is, however, found in a much larger proportion of the forms 

 studied. The measurements were taken from the middle of the 

 articulating surfaces as in some cases the outline of the bones 

 is very irregular. All of the Cetacea, as far as I have exam- 

 ined them, have the radius at least a little longer than the ulna 

 and in some forms, e. g., Phoccena, it is very noticeably so. In 

 the majority of the reptilian forms studied the radius was found 

 to be longer than the ulna ; in some cases they are of equal 

 length, but none were found in which the ulna exceeded the 

 radius. In the hind limb the same results were obtained except 

 that in Geosanriis the fibula is somewhat longer than the tibia. 

 It does not necessarily follow from the above that an angulation 

 of the limb is always produced by the longer radius or tibia. 

 In some cases the effect is verv noticeable and in at least one 



