20 MK. E. LEONARD GILL ON THE PERMIAN 



been fossilised under quiet conditions ; the parts are rarely much 

 disturbed and practically never scattered. On the whole this is a 

 disadvantage. The scales and the few strong bones (frontal and 

 angular for example) are often well preserved, but most of the 

 bones are delicate, and as tliey are crushed one upon another it is 

 usually ver}^ difficult to make out their boundaries. In the case 

 of the palate it is practically impossible. The distal parts of the 

 fin-rays, too, are hardly ever shown satisfactorily. It is only 

 through having abundant material for comparison that it has 

 been possible to m.ake out even so much of the skeleton as is here 

 described. 



Deccription of the Genus. 



The descriptions already given by Kirkby, Traquair (1877, 

 pp. 563-4), and Dr. A. Smith" Wood ward (1895, p. 51), together 

 with the restorations and other figures accompanying this paper, 

 render it unnecessary to go into much detail as to the main 

 external features characterising Acentrophorus. The general 

 shape (text-fig. 1), including the excavation of the back along 

 the base of the dorfal fin, is much as in Lep)idoUis. except that 

 the snout is bluntei'. To judge by the number and appearance 

 of the specimens that have been compressed dorso-ventrally, the 

 body Avas fairly rounded. The scales, compared with those of 

 most Semionotids, are thin ; they overlap deeply, nearly half of 

 each scale being covered by the one in front of it. Their only 

 ornament is a series of concentric lines near the hinder margin ; 

 these are presumably annual growth lines, and as many as five 

 can sometimes be counted. The absence of a row of dorsal ridge- 

 scales with projecting points is the character upon which Traquair 

 separated , the genus from Semionotus \ in place of points these 

 scales often even show a concave hind border. But the most 

 striking characteristic of the squamation is the great though 

 perfectly graduated variation in the size of the scales on diflerent 

 regions of the body, and particularly the great depth of the anterior 

 scales of the lateral line. In shape the lateral line scales are not 

 unlike those of Dapedms. The lateral line can usually hardly be 

 followed on the outer surface in A. varians and alius, except by 

 the notching of the scales. In A. glapliyms it is more prominent ; 

 and on the inner face of the scales it is marked hj a conspicuous 

 gxooYQ in all the species. Peg-and-socket articulation is well 

 developed on the inner face of the anterior scales of the upper part 

 of the flanks (text-fig. 2, A) ; but it is absent in A. varians from 

 the smaller scales of the lower flank, and dies out gradually even 

 in the upper scales as they are traced backward towards the tail- 

 shaft (text-fig. 2, B, C, D). , In ^. alius and A. glapliyrus the 

 scales have on the inside a low rounded ridge, which is continued 

 above into the peg (text-fig. 2, E), and in these species the peg- 

 and-socket articulation extends almost or quite to the tail. In 

 all the species the scales on the jugular region axe very small and 

 roundel (text-fig. 7, sc). As in many other Semionotids, there 



