406 ROY L. MOODIE 



tirely around the periphery of the bone. On one side of the 

 bone there are three vascular openings of some size as well as 

 many smaller ones. In adult bones all of the characters disap- 

 pear, the groove gradually being forced toward the distal end, 

 where it may persist for some time, the cavity, canal and fora- 

 men being gradually obliterated. 



Fig. 5 An embryonic plesiosaurian propodial, from the Cretaceous of Kansas. 

 This bone shows all the growth characters discussed in the present paper. X 1. 



The groove on the edge of the bone can only be explained 

 on the assumption that there are two ossific centers for the pro- 

 podium, and that between the extension of these the cartila- 

 ginous endochondrium reaches the edges of the flattened bones. 

 In the bottom of this groove a capillary bud has pierced the peri- 

 chondral (periosteal) lamella and has invaded the endochon- 

 drium, thus forming the 'foramen.' Later piercing of this 

 vessel (periosteal bud) results in the formation of the 'canal' 



