922 



DR. R. BROOM ON THE 



according to Parker " the alisphenoids and ovbito-sjDlienoids appear 

 as chondrifications of the walls of the skull." In studying the 

 development of the marsupial skull some years ago, I found that 

 the alisphenoid has originally nothing to do with the walls of the 

 skull. It first appears as a short rounded rod lying outside the 

 trabecula and quite independent of it or of any other skeletal 

 structure. In its relations it seems exactly to correspond to the 

 miiddle part of the cartilaginous bar on which the pterygoid bone 

 develops in Lizards and S'phenodon. In the majority of Lizards 

 this middle part gives rise to the epipterygoid or columella cranii. 

 When, as in Chaviceleon, the epipterygoid is rudimentary, the short 

 bar which forms its base is almost exactly similar in structure and 

 relations to the bar from which the alisphenoid developes in 

 Mammals. One therefore seems driven to the conclusion that the 

 epipterygoid and the alisphenoid are difierent developments of 

 the same element. And this conclusion seems borne out by 

 comparative anatomy, foi- we find that most Reptiles have either an 

 epipteiygoid or an alisphenoid, but never both. In Lizards we 

 find an epipterygoid, but never an alisphenoid : in Snakes an 

 alisphenoid, but never an epipterygoid. 



There seems little doubt that the epipteiygoid is the early type 

 of development. "We find, for example, in the primitive Proco- 

 lophon a columella cranii almost exactly like that of the lizard. 

 In the Therocephalians the columella? are long and slender, but 

 usually flattened. In the Anomodonts they are slender, but 

 rounded. In the Cynodonts we find no longer the columella 

 cranii, but in its place a broad fan-shaped alisphenoid. The 

 Cynodont alisphenoid further diflfers from the columella cranii 

 of the earlier forms in having the lower part well developed and 

 replacing the backward extension of the pteiygoid. In the 

 mammal the alisphenoid diflfers from that of the Cynodont mainly 

 in having the 2nd and 3rd branches of nerve v. passing through 

 it instead of behind it. 



[^Note hy Editor. — In Dr. Broom's memoir as presented to the 

 Society thei'e followed here a discussion of the quadrate and 

 tympanic, illustrated by two diagrams. A recent discovery made 

 by Dr. Broom has considerably modified his views, and he has 

 asked leave to withdraw the paragraphs omitted here until he 

 has time to work out and present to the Society in a fuller 

 form the bearings of his new facts. — August 11th, 1911.1 



Ancjular. 



The angular is found in all mammal-like reptiles. It is large 

 in the Dinocephalians, Anomodonts, and Therocejjhalians, but 

 comparatively small in the Cynodonts. In Mammals there is a 

 small splint-bone on the lower side of Meckel's cartilage which is 

 probably the remains of the angular. In Omiihorhynchiis there 

 appear to be two splint-bones, one being probably the surangular. 



