STRUCTURE OF PLESIOSAURIAN PROPODIAL 409 



hollow limb bones. Why activity should result in the produc- 

 tion of osteoclastic or other osteolytic elements is uncertain, 

 but the evidence that such was the case is clear. 



From the above discussion it is clear that the growth of bone 

 followed precisely the same lines in ancient Mesozoic times 

 as prevail today. The bone was doubtless preformed in car- 

 tilage, though no definite evidence of this is yet available. The 

 osteoblasts arranged themselves first around the periphery and 

 formed the perichondral (primary periosteal) lamella of bone, 

 which was later pierced by a 'periosteal bud,' either at the time 

 of formation of the bone or later by a process of erosion. The 

 osteoblasts doubtless migrated, in the plesiosaurs, as in modern 

 mammals through the opening made by the periosteal bud into 

 the cavity formed in the endochondrium and there formed the 

 secondary bone; which later became pierced by the Canales 

 ossificantes perforantes extending to the ends of the bones. 

 The cavity, canal and foramen later became filled up with 

 bone, owing to the absence of osteolytic elements and resulting 

 in the formation of a solid limb bone. Whether the foramen 

 and canal resulted in the formation of a Volkmann's canal, is 

 uncertain, but, since there are often evidences of more than 

 a single canal, it is quite probable that one or more of these 

 perforating canals through the primary lamella may have formed 

 such a structure. 



The growth of primary periosteal (perichondral) and the 

 secondary (endochondral) bone in the plesiosaurs was quite 

 distinct as is evidenced by specimens of embryonic and young 

 bones, in which it is clear that the formation of bone has been 

 produced by growth in different directions, resulting in a sharp 

 cleavage between the layers of bone. 



SUMMARY 



Growth characters among the Cretaceous plesiosaurs followed, 

 so far as the limb bones are concerned, essentially the "^ame 

 lines as do mammalian limb bones at the present time. The 

 characters present in immature plesiosaurian propodials can 

 be explained on the basis of embryonic characters of mammals. 

 SoUd limb bones, evidently only present in the more sluggish 



