Roy L. Moodie 449 
a remnant of the medullary cavity and a single small canal. On the 
external surface of the adult propodial (Fig. 7) there is no evidence of 
the cones or of the canals, but on sectioning an adult humerus (Fig. 9) 
the cones are seen to be represented by dense areas of tissue at the ends 
of the bone. The areas are without definite boundaries and merge 
gradually into the texture of the rest of the bone. 
The cavity which hes between the apices of the two cones is the medul- 
lary cavity. Its presence in the embryonic bone can be accounted for only 
Fig. 5. Fie. 6. Lime, ¥( 
Fic. 5. An immature plesiosaurian propodial showing foramen at a. 7/10 
natural size. After Williston. 
Fic. 6. Propodial of half-grown plesiosaur showing the foramen ata. % 
natural size. 
Fic. 7. An adult plesiosaurian humerus seen from the dorsal side. There 
are no foramina or cones evident. About 4 natural size. 
on the presumption that the ancestors of the plesiosaurs had hollow limb 
bones. The cavity is certainly a vestigial one. It is quite large in the 
embryonic bone (Fig. 4). There is a small cavity filled with calcite in 
the propodial of the half-grown individual (Fig. 8). In the adult bone 
(Fig. 9) the cavity has entirely disappeared. From these facts it is 
apparent that the cavity is a transitory structure and is on the same plane 
