ADAPTIVE MODIFICATIONS OF LIMB SKELETON 455 



paddle of Mosasatirtts Icinoinieri, though in part wanting, and of 

 IcJitliyosaurus acutirostris, also lacking the tips of the digits, 

 seem to be exceptions even to this and h^ve the first digit as 

 long as the others and perfectly straight. Perhaps a complete 

 limb might show this to be an error in judgment, however. 

 Frequently in the Cetacea the first digit is so reduced as to be 

 nearly or entirely wanting, and in the longipinnate Ichthyosaurs 

 it seems to be absent in all cases. This results naturally in 

 throwing the anterior border of the hand on the second finger, 

 and thus farther back. 



In a few instances the paddle is extended backward largely 

 by the broadening of the bones. Examples of this may be seen 

 in Mixosaurus, Ciinoliosmnms, etc. This matter will be alluded 

 to further under the head of distal dilatation. 



Distal Dilatation. 



A modification of universal occurrence among swimming 

 animals is the distal dilatation or broadening of the limbs in the 

 formation of paddles. At any rate the limb becomes broad in 

 proportion to its thickness. In certain long-limbed forms like 

 Megaptera among the whales or IchtJiyosaiirus acutirostris among 

 the reptiles the limb is evidently not greatly broadened, but it 

 is at least reduced in thickness until a thin paddle-like form is 

 attained. The incipient stages of this dilatation may be seen in 

 animals which have only recently taken up aquatic life and 

 which have not yet lost their land-relation. Evidently the first 

 part of the limb to show this broadening is the most exposed 

 part, /. e., the hand or foot. Later the process continues up 

 the limb until finally all the bones of the limb may become 

 affected. The humerus and femur may even become involved 

 in this change clear up to the articulation. 



In all but a few cases the point of greatest breadth remains in 

 the digits. The most common method of increasing the breadth 

 of this region is found in the spreading of the digits. This is 

 shown by practically all aquatic mammals and by aquatic reptiles 

 generally except the Plesiosaurs and the Ichthyosaurs where 

 spreading of the digits is rare and slight. Again, the breadth 



