STRUCTURE OF PLESIOSAURIAN PROPODIAL 407 



and the final invasion and spreading out of the arteries results 

 in the formation of the medullary cavity. From this medul- 

 lary cavity canals lead out to the extremities of the bone, as 

 will be immediately described. 



On the ends of the propodials of the immature Ogmodirus 

 martinii, are well-developed rugosities which take the form of 

 miniature volcanoes. (Plate 1, figs. 2 and 4). They are dis- 

 tributed over the entire articular surface of the bone. These open- 

 ings and mounds are, doubtless, explained on the basis of bone 

 growth in mammals. Bidder ('06) has offered an interesting 

 explanation for the formation of epiphyses in mammals, by 

 the migrations of the osteoblasts through special vascular canals 

 (Canalis vasculosis perforans) which traverse the space be- 

 tween the medullary cavity and the cartilaginous caps at the 

 ends of the limb bones. This condition is indicated diagramma- 

 tically for the plesiosaurs (text fig. 6), which is based on a study 

 of broken and sectioned plesiosaurian limb bones. The peri- 

 phery of the medullary cavity in the femur of Ogmodims is 

 very irregular and leading out from the sharp-pointed embay- 

 ments are the Canales ossificantes perforantes, as is indicated 

 in text figure 6. The blood vessels, and with them the migrat- 

 ing young mesenchymal cells — the osteoblasts — may have 

 traversed these canals through the previously formed bone to 

 enter the cap of articular cartilage. The action of the bone- 

 forming elements was not, in this case, to produce new osseous 

 growths (epiphyses) but simply resulted in the extension in 

 the length of the bone. The mounds on the ends of the propo- 

 dials look as if bony matter had been poured out of the small 

 opening to harden around the orifice, exactly as lava does around 

 the opening of a volcano. What really happened was that the 

 osteoblasts, if they form bone, arranged themselves on the 

 borders of the openings of the Canales ossificantes perforantes 

 and there formed the bone which resulted in the extension in 

 length. 



The liiTib bones of the plesiosaurs, ichthyosaurs, mosasaurs, and 

 some of the larger dinosaurs are sohd, as they are in the modern 

 manatees. This condition is, of course, due to the absence of 

 an osteolytic element (presumably the osteoclasts) or the pres- 



