450 Reptilian Epiphyses 
as the persistence of the gill clefts in the mammalian embryos. The 
ancestors of the plesiosaurs are yet to be discovered. When they are 
brought to light they will probably be found to have hollow limb bones. 
The canals and the foramina of the propodials are not so readily 
accounted for. The presence of canals in the middle of a limb bone is a 
very remarkable character and is without parallel among the known verte- 
brates. The canals are probably for the passage of blood vessels, but why 
Fia. 9. Fic. 10. 
Fic. 8. A section through the plesiosaurian propodial shown in Figure 6. 
A= Endochondral bone. B=Medullary cavity filled with calcite. C= Peri- 
chondral bone. % natural size. 
Fia. 9. A section through the humerus of an adult plesiosaur. 1% natural 
size. 
Fig. 10. Proximal portion of a young plesiosaur propodial showing three 
foramina. Seen from the edge. 1 1/10 natural size. 
they should be so large and why they should disappear in the adult, is 
inexplicable. The nutrient foramina, if such they are, have been observed 
in many specimens. Williston describes four in the embryonic propodial, 
there are three present in a more mature bone before me (Fig. 10), one 
