458 OSBURN 



advanced cases they may be broadened almost to the point of 

 the articulation with the girdle. The only completely aquatic 

 forms which show no modification of the humerus in this way, 

 as far as I know, are the Sirenia. This lack of modification 

 may be accounted for by the peculiar way in which the paddles 

 are used by these animals when feeding or resting in shallow 

 water and in holding the young. The femur of Gcosaiirus shows 

 little or no dilatation and remains very long, though in absence 

 of tuberosities, in curvature and in other ways it is evidently 

 natatory. The humerus of Geosaurus is much more progres- 

 sive and is greatly shortened and broadened. In the Cetacea 

 without exception the greatly shortened humerus is also notice- 

 ably widened, especially in PJioccBua and Globiocepliahis. In the 

 Plesiosaurs the humerus and femur are longer than is usual 

 among aquatic forms, but the dilatation is very evident. In 

 Cinwliosaiiriis trocJianteriis and C. portlandicus the ''pisiform" 

 and its homolog of the hind limb find room to articulate with 

 the propodial beside the greatly widened epipodials, and C. 

 eurymerus is even more expanded than these, being the only 

 case, as far as observed, where the broadest part of the paddle 

 is the distal end of the humerus. The extreme of modification 

 is found among the Mosasaurs and Ichthyosaurs. In the 

 former it is always marked and reaches a climax in the hand 

 of Platecarpns, Plioplatccarpiis and Clidastes. According to 

 Williston the humerus of Platecarpiis is the most modified, its 

 width distally being nearly equal to its length. The Ichthy- 

 osaurs are equally modified, even in the early Triassic forms 

 (cf. Mixosaurus, Toretocnemiis^ Merriamia and Shastasaiirus). 

 The humerus of Shastasaiirus^ according to Merriam, is the 

 most specialized known — "the shortest propodial segment 

 known in the limb of an Ichthyopterygian " — and a very nar- 

 row notch on the anterior border represents the " last trace of 

 a shaft of a * long bone.' " 



In the marine Chelonia there is formed a very good paddle 

 and the distal dilatation of the whole limb is very evident. The 

 individual bones do not show it to any extent, however, as far 

 as observed. The seals are in the same condition, and the 



