318 PllOF. U. G. SEELKT OK SIMILITUDES OF 



a sacrum. The earlier caudal vertebrae of both have transverse 

 processes jutting horizontally from the side of the centrum, and- 

 chevron bones between the centrums beneath. But in Cetacea 

 the transverse processes on the centrum are common to the 

 later dorsal vertebrae, and the neural arch and the chevron bones 

 do not persist to the end of the tail, and the later caudal vertebrae 

 of Cetaceans become singularly modified in form. As in Por- 

 poises, the dorsal centrum is usually rather longer than the early 

 caudal centrum. The dorsal ribs are similar to those of Cetaceans ; 

 but the sternal ribs of a Plesiosaur are unlike those of any 

 mammal. 



The pectoral g-irdle of Plesiosaurus has nothing in common Avith 

 that of Cetaceans or any mammal beyond a distant resemblance 

 to that of the Monotremes. 



The humerus of Phocana differs from that of Plesiosay/pus 

 chiefly in its shortness and the less constriction of the bone below 

 the proximal articulation. And the position of the limb on the 

 body appears to have been different ; for in Cetaceans the prox- 

 imal trochanter is anterior, while in Plesiosaurus it appears 

 to be exterior. The points of resemblance are in the proximal 

 and distal ends being in the same plane, in the side-to-side com- 

 pression of the distal end, which similarly has two flat articular 

 surfaces which meet at an angle, in the proximal end termina- 

 ting in a hemispherical articulation on one side of the bone and a 

 large trochanter on the other, though they seem to be on opposite 

 sides. 



The ulna and radius are relatively to the humerus much 

 longer than in Plesiosaurus. The radius is a compressed bone 

 with flattened articular ends in both groups ; but the Porpoise 

 differs in having the distal end the wider — while in Plesiosaurs 

 the proximal end is the wider when the ends differ in width, and 

 the anterior and posterior margins of the bone are both con- 

 cave. The ulna differs in being much wider and usually reniform in 

 'Plesiosaurs, so that the posterior margin is convex and the an- 

 terior concave, while in Cetaceans the bone is long and narrow, 

 concave behind and usually straight in front: it has a small 

 olecranon process. When the olecranon is developed in Plesio- 

 saurs, it always persists as a distinct ossification. 



The carpal bones are so similar in the two groups that they 

 cannot be distinguished from each other. In both they are flat, 

 compressed, subhexagonal, or irregularly ovate ossicles. The 



