456 . OSBURN 



of the bones of the digits may be increased sufficiently to make 

 this portion of the Hmb the widest. In the Sirenia the width of 

 the hand is in part due to this, and in Plesiosaurs where the 

 hand is widened and in some Ichthyosaurs the width is entirely 

 due to the great breadth of the digits. In many Ichthyosaurs 

 and rarely in the whales the hand or foot may be widened by 

 the addition of one or more extra digits, a process known as 

 hyperdactyly. Kiikenthal has shown this to come about in Be- 

 luga, the white whale, as the result of the splitting of the fifth 

 digit and there is reason in the view that hyperdactyly has come 

 about in the same way in Ichthyosaurs. In Cetaceans never 

 more than one extra digit is formed, but as many as four extra 

 digits, or nine in all, are known in Ichthyosaurs. These without 

 exception are found on the posterior border of the paddle. It 

 is possible that still another method of broadening the hand 

 may have been the separation of the bones of the digits by em- 

 bedding in cartilage. In Ichthyosaurus ingens, and especially in 

 Baptanodon, this seems true. 



In a few cases only, the carpal and tarsal region is found to 

 be the widest part of the limb, though this region is usually 

 somewhat broadened. In Mixosaurus this is exceptionally well 

 shown. In some whales as Globiocephalus or PJioccena this 

 region is the widest part of the limb but here the fifth finger has 

 moved up alongside of the carpus, and the carpus itself is not 

 greatly broadened. In some instances the breadth of this re- 

 gion appears to be much increased by the cartilaginous separa- 

 tion of the bones. Thus in Delphijiapterus, PJwccena, etc., the 

 carpal region is quite broad while the bones are small and em- 

 bedded in cartilage. The same thing is evident in Baptanodon 

 among the Ichthyosaurs. A cartilaginous extension of the 

 posterior border of the carpus, materially widening this region, 

 is known among the Cetacea. 



The epipodials tend toward great breadth and become short- 

 ened about in proportion as they become widened. Among 

 the forms here dealt with they are found the least modified in 

 the genus Balcenoptera, among the Cetacea, where, although 

 they are somewhat flattened, they are about twice the length of 



