PERMIAN FISHES OF THE GENUS ACENTROPHORUS. 19 



4. The Permian Fishes o£ the Genus Acentrophomts*. By 

 E. Leonard Gill, M.Sc, The Royal Scottish Museum, 

 Edinburgh. 



[Received June 6, 1922 : Read February 6, 1923.] 

 (Text-figures 1-16.) 



Tiitroditctoiy. 



As the earliest known members of the family Semionotidae and 

 the only Paljeozoic Actinopterygians of a higher grade than the 

 Chondrostei, the small fishes named Acentrophorus by Traquair 

 (1877), found in the Permian of the County of Durham, have fre- 

 quently been referred to and discussed in palfeontological writings. 

 An examination of the abundant material in the museuins at 

 Newcastle and Sunderland showed that it would be possible to 

 add considerably to what was hitherto known of the structure of 

 these fishes, and the main results of such an examination are 

 here given. 



The first species of Acentrophorus to be named and described 

 was A. glaphyrus, which Agassiz published (1835) as a species of 

 PalcBoniscns. It is found in the Marl Slate, the lowermost bed 

 of the Permian series in Durham and Northumberland, but as it 

 is rare and not well preserved it has furnished little information 

 as to the structure characteristic of the genus. A much more 

 abundant and better preserved fish is A.varians from the Upper 

 Magnesian Limestone. It was described as Pcdceoniseus varians 

 by J. W. Kirkby (1862 and 1864), who at the same time described 

 two much scarcer species occurring in the same beds, '''' Palce- 

 oniscus" altus and "P." abbsi. Reasons are here given for 

 doubting whether the species abbsi ever ha.d a real existence. 

 In 1873 Liitken pointed out that these fishes were wrongly 

 assigned to Pcdceoniseus: and in 1877 Traquair established for 

 them the genus Acentrophorus. 



Kirkby 's account (1864) includes a careful description of the 

 general proportions, the squamation, and the fins. Traquair 

 added many details, especially in regard to the osteology of the 

 head. In a few minor points, however, the material at his dis- 

 posal evidently led him astra_y, and it does not appear to have 

 shown him anything of the teeth, the axial skeleton, or the bones 

 supporting the fins. Few figures of the fishes now included 

 under Acoitrophorus have been published, and none give a very 

 clear representation of their structure. The specimens, from 

 both the Magnesian Limestone and the Marl Slate, seem to have 



* Communicated by D. M. S. Watson, M.Sc, F.R.S., F.Z.S. 



2* 



