Apr. 1903. NorrH AMERICAN PLESIOSAURS —WILLISTON. 45 
been covered with cartilage below, leaving a free, convex, ventral 
surface, elongate oval in shape, including the emarginations, limited 
by a distinct ridge or angle. The striation of this thinned margin 
points to a covering of cartilage. The anterior emargination of the 
bone is narrow and deep, while the posterior one is broader and even 
deeper. It differs markedly from the corresponding bone of P/. mudgec 
(see Pl. XXVII), in its less broad and deep emarginations. An 
episternum which I provisionally refer to the species 77. anony- 
num, herewith figured, resembles that of D. osborni more closely. In 
all three, perhaps belonging to distinct genera, it will be seen that 

Fig. 10, 
1, Episternum and clavicles of Wuraenosaurus ; 2,3, the same of Plestosaurus, from above and 
below. After Seeley. 
