448 Reptilian Epiphyses 
one described and figured by Williston (8). I have studied this propodial | 
and it offers some very peculiar characters. As described by Williston 
there are two cones of osseous matter whose apices do not quite meet at 
the center but are separated by a small medullary cavity into which four 
canals from the outside enter. 
The cones of this embryonic propodial (Fig. 3) are represented in the 
diagram (Fig. 4). Williston says: “The... . specimen discloses 
these epiphyses (cones) with a smooth rounded surface . . . . the outer 
NS 
\\ \\ 
YAS 
Fie. 3. Fie. 4. 
Fic. 38. Embryonic plesiosaur propodial. Foramina at a and b. ¥, natural 
size. After Williston. 
Fic. 4. A section through the embryonic propodial shown in Figure 3. 
A —Foramen of the nutrient canal. B—=Medullary cavity. C—Cone of the 
endochondral bone. D=—Perichondral bone. 44 natural size. 
part peeling away as does the bark from a tree.” The structure of this 
propodial is especially dense and the same has been described for many 
other bones of young plesiosaurs. The denseness of the tissue is one of 
the peculiar characters of these cones and persists throughout the life of 
the individual. I have made sections of several propodials to determine 
the fate of the cones and find them clearly marked in the propodial of a 
half-grown individual (Figs. 6 and 8). In this bone there also persists 
