Anomodont SJcull. 79 



sented in the suborbital regions are due to the diflperent (h^grees of 

 development of the squamosal and maxillary l)ones. 



III. — The Preparietal. 



In all skulls of Lystrosmirus that are at all well preserved 

 there is seen immediately in front of the ])arietal foramen a 

 moderate sized median bone. It articulates with the paired fron- 

 tals in front, and tits in between the paired parietalsl)ehind. It was 

 apparently first noticed by Huxley, and in the British Museum 

 Catalogue, 1890, it is referred to by Lydekker as a " Wormian 

 bone." By Seeley it is referred to as the " Preparietal " bone. It 

 was prol)ably the absence of a median suture where this bone is 

 situated that misled Owen into the idea that the parietal is single. 

 The relations of the bone are best seen in Seeley 's (*) figure of the 

 upper side of the skull of Mochlorhhiiis platyreps { = Lystr(h 

 !^au nis ]j?atycej)s). Whether it is a wormian bone or an element 

 of deeper significance it is at present difficult to decide. I am not 

 aware that it has hitherto been found in any genera but Lysfro- 

 sa/iiri(s and Gci'donia, but recently I have discovered that it is 

 l)resent in })ractically all the Anomodont genera. In Oudmodon 

 trigonicops it is (]uite distinct and of fair size. It extends from 

 the parietal foramen as far forward as the level of the posterior 

 l)order of the orbits, and it is about half as broad as one of the 

 frontals. Though it is thus a much narrower bone than in Lystro- 

 ^aiirus, it is relatively about as large, and it has exactly similar 

 relations to the frontals, parietals and parietal foramen. On look- 

 ing for it in other Anomodonts I found that a very small but 

 distinct preparietal occurs in Oudewdon gracilis. Here it might 

 almost be described as rudimentary. In Oudenodon truncatvs the 

 })i"eparietal is also distinct, and though relatively small it is by no 

 means rudimentary. In the Endothiodont genus Opisthoctenodon 

 there is also a fairly well developed preparietal lying in front of 

 the parietal foramen exactly as in Lystrosmirus. In Dicynodon^ 

 owing to the post-orbital bones usually forming more or less 

 distinct crests, and rendering it difficult to satisfactoi'ily remove 

 the matrix from this region I have been unable to clearly demon- 

 sti'ate the presence of a preparietal. In one small skull, however, 

 in which only the cast of the hones of the parietal region remains, 

 the ap})earances strongly indicate the presence of a distinct 

 preparietal. In Gordonia there is a median element lying mainly 

 in front of the parietal foramen, and which doubtless cui'i"('si)onds 



