34 MR, E. LEONARD GILL ON THE PERMIAN 



ill-founded. Text-figs. 1, 12, 13, 14, 15 illustrate the differences 

 distinguishing these species, and will be sufficiently supplemented 

 by the following brief notes : — 



Acentrophorus varians (Kirkby). (Text-fig. 1.) The type- 

 species and very much the most abundant. In spite of its name, 

 not more variable than many modern fishes, e. (/., trout, roach. 

 Specimens range in length from 5-11 cm. 



Acentro2}horus alius (Kirkby). (Text-fig. 12.) Yery much 

 scarcer than A. vatHans but occurring in the same beds. Differs 

 only in its deeper form and, as already stated, in the character of 

 the inner surface and articulation of the scales (text-fig. 2). 

 Length 4-8 cm. 



Acerdrophorus abhsi (Kirkby). Also from the Upper Magnesian 

 Limestone of Co. Durham. Supposed to be distinguished from 

 the other species by its extraordinarily lengthened form. Close 

 examination of the two or three examples in Kirkby's collection 

 seems to show that they are no more than distorted specimens of 



Text-figure 12. 



AcentropJiorus alius. Restoration. 

 About natural size. 



A. varians. Text-fig. 14 shows the head of the type-specimen. 

 It will be seen that the parts are dragged out to a great degree 

 longitudinally. The opercular apparatus has held together, but 

 is lying far behind its proper position in relation to the squamosal 

 and lower jaw, leaving the hyomandibular region widely exposed. 

 Similarly the squamosal, supratemporal, and post-temporal are 

 dragged apart, and the pectoral fin pulled from its connexion 

 with the shoulder-girdle. A similar distortion is apparent in the 

 whole length of the body, the scales hardly or not at all over- 

 lapping and in places separated altogether. Kirkby's figure 

 (1864, pi. 18. fig. 3) shows the posterior half of the body 

 separately ; in the actual specimen the two halves are parted by 

 a very shallow horizontal fault or slip-line which has carried them 

 some distance asunder, with a trail of scales as their only 

 connexion. The condition is difficult to account for, but it is 

 evident that movement took place in the surface layer of lime- 

 stone before it was quite consolidated but after the fish had 



