COMMENTS BY D. M. S. WATSON 981 



ossification in the cranial wall of a reptile. In Crocodilus the Gl-asserian 

 ganglion lies outside the cranial cavity in a small chamber, widely open 

 back and front and included by the pterygoid and "alisphenoid.'' In 

 Belodonts this cavity is exactly similar, but its outer wall is formed en- 

 tirely by an epipterygoid. 



Whether, as Oken (1811?), Parker, Baur (at one time), Broom, and 

 Fuchs believe, the epipterygoid is homologous with the alisphenoid is 

 much more doubtful. In my Monotreme skull paper, to be published 

 very soon [Phil. Trans. Eoy. Soc. London, ser. B, vol. 207, 191C), pp. 311- 

 374, 3 pis.], I have gone into the question very fully, and concluded that 

 the obvious reading adopted by these authorities is wrong in part. 



The mammalian alisphenoid is an ossification of the ala temporalis, 

 which spreads into the membranous cranial wall, which is not homologous 

 with the cranial wall of lizards, but lies outside it. Gaupp has shown 

 that part, at any rate, of the ala temporalis (great wing of the sphenoid 

 of human anatomy) is homologous with the processus basipterygoideus 

 of reptiles, amphibia, and fisli. Broom shows that its outer end is homol- 

 ogous with the pars palatina of the palato-quadrate cartilage. Hence, 

 from other reasoning to that above, the Crocodile "alisphenoid^' can not 

 be homologous with the true alisphenoid of a mammal. 



Hence I accept v. Huene^s name laterosphenoid for the "alisphenoid" 

 of the Crocodile and all bones shown to be homologous with it.^ 



The skull of the living Amphibia differs from that of Eeptiles in being 

 extremely platybasic — that is, in having no interorbital septum — the 

 lengthy brain-case extending forward to the nasal region and filling the 

 whole space between the parasphenoid and the roof of the skull. 



This condition in Amphibia is plainly secondary, depending on the 

 dorso-ventral flattening of the head, which is a characteristic aniphibian 

 advance. 



In the Carboniferous Pteroplax there is a large interorbital septum, 

 which supports the anterior end of the brain-case, just as does the largely 

 membranous interorbital septum of the lizards and teleosts. The gradual 

 flattening of the skull in large Amphibia (even in, say, Eryops and Capi- 

 tosaurus), together with some enlargement of the brain cavity, leads to 

 the gi'adual loss of the interorbital septum, the whole brain-case being 

 floored by the parasphenoid. 



Hence the characteristic "os en ceinture" form of the frog's spheneth- 

 moid depends on the actual shape of the skull, which is purely second- 

 ary, and in types with a distinct interorbital septum we should expect the 



5 Compare Williston's remarks helow, p. 985. — Editor. 



