Apr. 1903. North American Plrsiosaurs — Williston. 69 



folds of the skin, as was, evidently also the case, not only in the ich- 

 thyosaurs, but also in the mosasaurs. 



The largest bone beyond the humerus is the radius. It is 

 irregularly four-sided in shape; the longest and convex proximal sur- 

 face fits into the radial facet of the humerus; the shortest and non- 

 articular border, that adjoining the ulna, is emargiuated like its 

 opposing border to form with it a small foramen. Doubtless these 

 two emarginations represent the last vestige of the terrestrial type of 

 the t pipodial bones. The outer border of the radius, the second in 

 length, is non-articular and thinned, and has an acute angle proxi- 

 mally. The outer distal margin is thick for articulation with the 

 radiale of the carpus. The inner distal border, next to the shortest 

 of the four, is for articulation with the mediale. 



The ulna is next in size to the radius, and is irregularly six-sided. 

 The largest, proximal border, is convex, like that of the radius, and 

 joins the smaller facet of the humerus. The distal border has three 

 facets of nearly equal length, and joining each other in nearly equal 

 angles. These facets are for the mediale externally, the ulnare in the 

 middle, and for the ulnar supernumerary, internally. On the inner 

 side there is a longer, but thinner border for articulation with the first 

 epipodial supernumerary, while the outer border is emarginated like 

 the opposing border of the radius. 



The next bone of this row, articulating with the humerus, and of 

 considerable size, is one of doubtful homology. I will call it the first 

 epipodial supernumerary. If one follows Marsh in his views of these 

 corresponding bones in Ba/>ta>iodo>i, then this bone is the ulna, and 

 the one here called the ulna is the medial carpal. But this inter- 

 pretation is very doubtful. Zittel thinks this bone in the ichthyosaurs 

 is the pisiform, while Woodward calls it the sesamoid; but I doubt 

 these interpretations as well. Among the cetacea, the mosasaurs, 

 ichthyosaurs and plesiosaurs there is a reduplication of bones, which 

 have been variously explained as a splitting of the phalanges, either 

 directly or through the intervention of the epiphyses. The latter 

 view is hardly possible, since there are no epiphysial ossifications in 

 most if not all these reptiles, and besides, there would not be enough 

 epiphyses on the normal digit to furnish the great number of phalanges 

 seen in some of the forms. The extra digits in some ichthyosaurs 

 are also explained by the longitudinal division of the normal phalanges; 

 but I cannot believe that this is the correct explanation. 1 believe 

 that the supernumerary digits, phalanges, epipodial or mesopodial 

 bones found in such species as this, as well as in most other American 

 plesiosaurs, represent entirely new ossifications in cartilaginous 



