SKULLS OF CYNODONT REPTILES. 921 



Versluys has discovered what he believes to be a large para- 

 sphenoid in Deronochelys in addition to the vomer and in no way 

 connected with it. If this determination be correct, it will 

 probably turn out that the Chelonian vomer is after all a pair of 

 prevomers fused. The early develo^^ment of the Chelonian vomer 

 has not, so far as I am aware, ever been examined, and in my paper 

 on the reptilian and mammalian vomerine bones I spoke very 

 guardedly on the subject. So far as we know, the Chelonian 

 vomer is always a median unpaired bone. But if it be a true 

 vomer, what of Yersluys' supposed parasphenoid ? Fuchs has 

 shown that in Chelone the basisphenoid is ossified by a large 

 irregular exostosis on its under side, and that this exostosis bears 

 relations to the pterygoids very similar to those which the para- 

 sphenoid of Yersluys does. In the light of the observations of 

 Fachs, I think it must be concluded that the supposed parasphenoid 

 in Dermochelys is entirely a development of the basisphenoid, and 

 not the homologue of the parasphenoid of other reptiles. 



Gaupp and Fuchs have both apparently discovered a rudimentary 

 ossification behind the vomer in Chelonians which they believe 

 to be a true parasphenoid, and Fuchs has discovered what he 

 believes to be a rudimentary parasphenoid in Didelphys. The 

 situation of these rudimentaiy ossifications is undoubtedly that of 

 the parasphenoid, but they are also in the region normally occupied 

 by the vomer in Mammals. When a bone which occupies one region 

 in an ancestor comes to occupy a somewhat different region in 

 a descendant through a portion of the bone becoming aborted, it 

 is by no means uncommon that rudimentary ossifications can be 

 detected in the region abandoned. The os carunculpe is undoubt- 

 edly the internasal process of the premaxilla in Ornithorhynchus 

 and Echidna, but though it is quite detached from the pre- 

 maxilla, it is nevertheless a j)ortion of the premaxilla. In the 

 case of the vomer, supernumerary ossifications appear to be not 

 uncommon both in front and behind. In Orycteropus there are 

 two small ossifications in front, apparently not prevomers, but 

 detached ossifications of the true vomer. Kitchen Parker seems 

 to have found them so commonly present that in some groups 

 he regarded them as the rule. Speaking of the condition in 

 Marsupials he says : " The main vomer is often relatively small ; 

 there is, nearly always, a pair of antei-o-lateral vomers .... and 

 large posterolateral, and other, or postero-medial vomers ; these 

 are very irregular and unsymmetrical in the young Cuscus 

 especially, in which I find ten vomerine bones." Parker's postero- 

 medial vomers are probably the ossifications regarded by Fuchs as 

 parasphenoids, and there seems no reason to regard them as of 

 any more morphological significance than the Wormian bones in 

 the human skull. 



AUs2jhenoid. 



Until recently the alisphenoid bone has been looked upon, 

 like the orbito-sphenoid, as an ossification of the cranial wall, and 



