ApR. 1903. NorrH AMERICAN PLESIOSAURS—WILLISYON. 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 emarginated 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 epipodial 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 w/va is next in size to the radius, and is irregularly six-sided. 
The largest, proximal border, is convex, hke 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 ae 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 Baptanodon, 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. I 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 
