Roy L. Moodie 451 
in another, and there is but a single foramen present in the element 
figured by Williston on Plate XXIII of his North American Plesiosaurs 
(Fig. 5). Kiprijanoff has figured an immature propodial (Figs. 12 and 
13) from Russia, in which there is a single foramen present. It is thus 
evident that the canals and their foramina are very variable in number 
and there is also a variation in their position on the bone. In the embry- 
onic propodial they open out on both the dorsal and ventral surfaces 
(Fig. 3). In the propodial shown in the photograph there is a single 
foramen evident on the dorsal surface (Fig. 6). In all other specimens 
with which I am acquainted the foramina open out on the edge of the 
bone (Figs. 5 and 10). From these facts it would appear that the blood 
Pie. 12: Fig. 13. 
Fie. 11. A longitudinal section through an immature propodial of a ple- 
siosaur showing the cones and the perichondral sheath. % natural size. 
After Kiprijanoff. 
Fic. 12. An immature propodial showing foramina. Seen from the edge. 
% natural size. After Kiprijanoff. 
Fig. 13. Same bone seen from the dorsal aspect. °%4 natural size. After 
Kiprijanoff. 
vessels which the canals contained were veins and not arteries. That 
arteries are among the more constant structures in the body of the verte- 
brates is well known among anatomists, and where inconstancies occur in 
blood vessels it is usually among the veins. I have made sections 
of several immature propodials in the hopes of finding some clue to the 
meaning of the foramina and canals, but nothing can be stated here other 
than that they are present in the young and absolutely disappear in the 
adult. The bone fibers around the foramina in one section are seen to 
have a spiral arrangement (Fig. 14). 
