324 Mr. D. M. S. Watson on 



and epipterygoid of Reptiles are formed in connection with a 

 cartilaginous 'Mnlage." 



If this view be accepted, it folloivs that the mammalian 

 pterygoid is not homologous Avitli the reptilian pterygoid, 

 hut with the reptilian transverse bone, a view already held 

 by Seeley. 



It remains to discuss the remarkahle bone developed in 

 Cynodonts and other Therapsids in connection with the 

 tympanum, which has been called by Seeley a stapes, a 

 rudimentary cochlea, and a tympanic, and by Broom con- 

 sistently a tympanic. 



This bone is not present in its natural position in any of 

 the skulls of higher Cynodonts which I have been able to 

 examine ; it is^ however, preserved in position in the type 

 skull of Thrinaxodon liorhinns, Seeley, and displaced in one 

 of Seeley's Gomphognathiis skulls. It has also been figured 

 in Cynognathus j)l(ityceps\iy Broom. 



In Gomphognathus it is a narrow, straight, thin bone 

 forming part of a cylinder, anrl, according to the evidence of 

 the other specimens, articulating at the inner end with the 

 pei'iotic (or basisphenoid ?) just below the fenestra vestibuli, 

 and at its outer end with the quadrate. 



I can see no evidence that it took any part in the support 

 of the membrana tympani, for had it done so the membrane 

 would have been twisted, part being horizontal and part 

 vertical, a position in itself improbable and not supported by 

 the appearances of the rest of the region. There can be 

 little doubt that the function of this bone is to protect the 

 tympanic cavity from pressure from beneath, a necessity in 

 an animal with a very wide gullet in which the cavity was 

 exposed to the pressure of food. 



I know of nothing in Reptiles except the immediate 

 relations of Cynodonts which can be compared to this bone ; 

 but it is instructive to compare it with the tympanic and 

 entotympanic of Mammals. 



In all the more primitive Mammals with which we are 

 acquainted the function of the tympanic is the support of the 

 membrana tympani. It never takes any part in the 

 strengthening of the floor of the tympanum. When increase 

 in size and other alterations rendered the provision of a 

 bulla necessary, it was provided by three distinct bones — in 

 Marsupials by the alisphenoid, in the Tupaiids by the ento- 

 tympanic, and in the majority of higher Mammals by the 

 tympanic. 



