404 ROY L. MOODIE 



The writer believes that an adequate explanation of these 

 interesting conditions is to be found in the developmental his- 

 tory of mammalian long bones. 



The embryonic characters in developing long bones of mam- 

 mals are so well known that little need be said here concerning 

 them. Szymonowicz ('02) has figured in a developing long 

 bone of a mammal an opening which he terms 'periosteal bud/ 

 similar in all respects to the opening in the edge of the plesio- 

 saurian propodial. In both cases a canal leads from the fora- 

 men into the medullary cavity. A similar observation is re- 

 corded in the text-book of histology by Bohm, Davidoff and 

 Huber (p. 117, fig. 85) for a lizard embryo, and showing a much 

 earlier stage in the development of the canal. These observa- 

 tions are readily confirmed on embryonic material; the pig 

 showing the characters in a well-developed manner. 



That this canal and opening persist for some time in foetal 

 life is indicated by Jackson ('04) where he has figured and de- 

 scribed a similar but more advanced condition in the tibia of 

 a three day cat. Through the opening in the edge of the bone 

 pass the bloodvessels supplying the medullary cavity, as well' 

 as the osteoblasts and the marrow-forming elements. It is 

 very interesting to observe the persistence of this canal and 

 foramen in mammals for that is exactly the condition in the 

 immature plesiosaur referred to above (Ogmodinis martinii) 

 as well as in all embryonic propodials of the plesiosaurs; 

 with the important difference that this embryonic feature 

 was retained much longer in the ancient plesiosaurs than 

 it is in the modern mammals. There is no reason for 

 doubting that the same processes of bone formation went on 

 in Cretaceous times as are goin^g on now, so the comparison is 

 a safe one. 



In all, or nearly all, young and embryonic bones of plesio- 

 saurs, which are fairly common in the Cretaceous deposits of 

 Kansas, there occur the above mentioned foramina, canals, 

 cavities, and a well-marked groove, extending, in some cases 

 entirely around the bone, and often persisting until late in life, 

 being known to occur in an adult limb bone of Polycotylus. 

 A representative embryonic propodial is figured herewith (text 



