470 OSBURN 



cases finally lost. The Mosasaurs show an interesting series in 

 this respect with stages marking all the steps from the Lacertil- 

 lian type of carpus to that of Tylosaiiriis prorigej' which retains 

 only one small round ossicle in the mesopodial region. The 

 Ichthyosaurs also show many intermediate stages between the 

 interlocked bones of the early Jurassic species and the carti- 

 lage-embedded ones of Baptanodon. In the Cetacea such forms 

 as DelpJiinapteriis^ PhoccEna, and BalcEua are especially instruc- 

 tive. The ends of the epipodials and the distal end of the pro- 

 podials may also become covered with cartilage to the extent 

 that they are more or less evenly rounded, for all bones tend to 

 take this rounded form when they become so embedded in 

 cartilage that they are not affected by the impact of any neigh- 

 boring bones. 



Loss OF Movable Articulations in the Limb. 



All necessary movements of the natatory limb can best be 

 made at the point of attachment of the limb with the girdle, all 

 movements are of the limb as a whole, no flexing or torsional 

 movements within the limb are required, no motion of certain 

 digits or other special parts is necessary. Hence there is in all 

 swimming limbs the tendency toward the reduction of all mov- 

 able joints, the final result being the production of a more or 

 less flexible paddle without power of special movement in its 

 mdividual parts. Such we find it in the Cetacea, Ichthyosauria, 

 Plesiosauria and Mosasauria and probably in the Thalatto- 

 suchia, and partially in the Sirenia, Pinnipedia and marine Che- 

 Ionia. This may be attended by anchylosis of certain bones as 

 in M.onodoji where humerus, radius and ulna are solidly united, 

 or in Manatus where radius and ulna are anchylosed, but this 

 never occurs except with proximal elements. More generally 

 the bones are merely united by or embedded in cartilage, as 

 this admits of a certain flexibility. Li many Ichthyosaurs and 

 Plesiosaurs the bones interlock quite closely without the inter- 

 position of a noticeable amount of cartilage, but in the latest 

 forms of both these groups the cartilaginous embedding of the 

 bones becomes evident. The marine Chelonia, which go on 



