334 Correspondence. 



Salter, " like that genus," is it of any value whatever in determining 

 the horizon of the Boch in which it was found? 



If difficulty and doubt must exist in determining the affinities of 

 detached scales, much more is this the case with spines. On the 

 fragment of Limestone already referred to, are one or two imper- 

 fectly preserved spines which, if found by themselves, would have 

 been thought to belong to a Biplacanthus, nearly allied to D. 

 longispinus ; they undoubtedly differ in form from all the spines of 

 that fish, but not more so than these differ from each other : in 

 short, the spines of Carboniferous Fishes occasionally so nearly re- 

 semble those from the Old Eed Sandstone that, unless they possess 

 some very marked features, I should think, it far from safe to rely on 

 such fragments as good evidence. But supposing the specimens do 

 possess peculiarities, sufficiently marked, for referring them, with 

 certainty, to known genera, is this enough ? 



Some genera are confined to a comparatively narrow horizon, 

 while others range widely in this respect. The genus Acanihodes 

 is found in the lowest beds of the Old Eed Sandstone ; in its middle 

 division, in the Carboniferous, and in the Permian formations. 

 Even that very peculiar genus Pteriehthys is found in the middle and 

 upper Old Eed Sandstone, and in all probability only terminates its 

 existence in the Lower Carboniferous, and so with many others. 



Oddly enough this appears to be the case with the genera to 

 which Mr. Pengelly's specimens are somewhat doubtfully assigned. 



Of Phyllolepis, Agassiz, the founder of that genus says (V. G. E. p. 

 67) " Je connais maintenant deux especes de ce genre, dont I'une 

 provient du vieux gres rouge, Vanter de la houille," while Ctenacanthus 

 is as much, if not more, a Carboniferous than an Old Eed Sandstone 

 genus. 



If this genus will not determine the horizon, can these specimens 

 be with certainty assigned, to any known species ? 



In our lowest Forfarshire beds we have Acanthodes Mitchelli ; in 

 the Murrayshire nodules, and Caithness Flags, Ac. pusillus ; and in 

 the Caithness Flags also Ac. Peachi and Ac. coriaceus, all well ascer- 

 tained species, these formations representing an immensely extended 

 period in time : yet remove their spines from these Fishes, mingle the 

 spines together, and no one could tell the species to which any one of 

 them belonged. The scales, indeed, do vary, but this to so small an 

 extent that it is only in the best preserved specimens, and with the 

 use of highly magnifying powers, that the difference can be detected. 

 The same might be said of the Acanthodes from the Coal formation as 

 compared with those of the Old Eed Sandstone, only the former 

 being larger fishes have larger scales and spines. This is no doubt 

 an extreme case, but similar, if less striking examples are to be met 

 ,with in other genera. 



Is it, therefore, safe to put any, or at least implicit, reliance on 

 such fragmentary evidence ? 



I fear with most the verdict would be our Scotch one "Not 

 Proven." — I am, dear sir, very truly yours, Jas. Powrie. 



Reswallib, Fokfae, May 11th, 1867. 



