24 MR. E. LEONARD GILL ON THE PERMIAN 



much more satisfactorily preserved. Those of the cranial roof 

 are shown in outline in text-fig. 3. The pair of parietals together 

 roughly form a square. One well-preserved example of A . alius 

 shows two parietal ossifications on each side (text-fig. 4), hut as 

 other specimens have only one, this is presumably no more than an 

 occasional aberration. The parietals are flanked by " squamosals " 

 (pterotics, supratemporo-intertemporals) which have a rounded 

 expansion of the outer border anteriorly. The frontals are fused 

 into a single plate, the strongest bone in the whole skeleton with 

 the exception of the cleithrum ; it is nearly always found either 

 turned outwards clear of the head or folded down beneath it, 

 rarely crushed together from side to side. It is wide behind and 

 much narrowed in front, the very large orbits producing deep 

 excavations of its lateral margins. Three of the circumorbitals 

 adjoin each of these excavations. The nasals are so delicate that 

 though their extent is seen, their exact shape cannot be made 



Text-figure 3. 



Outline restoration of skull-roof of Acentrophorus varians. About one-aud-a-half 

 times natural size, cor., circumorbitals ifr., frontal ; m.tm., median temporal 

 plate; «a., nasals; joa., pai-ietals; ^.iwx., premaxillse ; p.iw., post-temporal; 

 sq., squamosal; s.tm., supratemporal. 



out, chiefly becaiise they are crushed down on the comparatively 

 sti-ong backward processes of the premaxillpe. In some specimens 

 there is a suggestion of a small median " ethmoid " and of 

 separate lateral ossifications (either prefrontals or adnasals). 

 The edge of what may be a postfrontal sometimes appears under 

 the anterior border of the squamosal. A pair of very large scale- 

 like post-temporals adjoin the hinder margin of the parietals and 

 squamosals, the suture being overlaid by a transverse pair of 

 supratemporals (extrascapulars). A smaller median plate (text- 

 figs. 3 and 4, m.im.) is wedged in between the post-temporals ; 

 it usually overlaps one of them, and is overlapped by the other. 

 It perhaps belongs to the supratemporal series. 



The large orbit is surrounded by a chain of about ten circum- 

 orbitals, of which those forming its posterior boundary are much 

 narrower than the rest. BetAveen the circumorbitals and the 

 preoperculum there were probabh^ thin cheek-plates, but their 



