Powrie — On the Genus Cheirolepls. 151 



he reduces the number of reputed species from the Carboniferous 

 formation from not less than 260 to 120, and of these 20 are new to 

 the British strata. This cause may affect fishes less than the more 

 lowly organised mollusca, but even in fishes it must always be 

 taken into account. 



I shall indicate only two of the many causes, which may seem- 

 ingly introduce very considerable divergence, where none really 

 exists. 



(1) The character and nature of the matrix in which the fossil is 

 imbedded. — This very considerably, and, perhaps, chiefly affects the 

 external markings of the more minute parts of the animal, — such as 

 the head-plates, scales, etc. As a good illustration of this, 1 may 

 mention the figures of the small Acanthodean Fishes, from Farnell, 

 in Forfarshire, (most accurately described by Sir Philip Egerton), 

 in the Tenth Decade of British Organic Remains, drawn by 

 Dinkel, than whom none has more experience, or better know- 

 ledge of such work. The scales of Acanthodes Mitchelli, and of 

 DiplacantJius {Ischnacanthus) gracilis, are figured with a granular 

 ornamentation ; better specimens prove them to have been perfectly 

 smooth. The ornamentation on the scales of CUmatius scutiger, as 

 thus given, is totally unlike that which their inner surface really 

 possessed, while the outer was perfectly smooth. No blame can 

 possibly be attached to the artist for this, the mistake being entirely 

 occasioned by the nature of the matrix in which these fishes were 

 preserved. 



(2) The state in which the body of the animal was when silted up 

 very considerably affects its apparent form. — Had decay advanced to 

 a certain stage, the generation of gases, and similar causes, tend to 

 give a very exaggerated form to the creature, while the pressure to 

 which it may have been subjected after being entombed,- must have 

 considerably increased its distortion. Especially is this the case with 

 most of the Palaeozoic Fishes, their dermal covering and appendages 

 being almost the only portions preserved. Hence the comparative 

 size of the head (from displacement of the head-plates), and depth of 

 the body, often seem to vary much in the same species ; the size of 

 the fijas, also, from being pressed together, and imperfectly pre- 

 served, in some appear smaller than they really were, while in 

 others they are exaggerated from being unduly pressed out. That 

 this is the case is attested by the fact that a confused mass of 

 scales, bones, spines, &c., is so often the state in which complete 

 Fishes are found. 



Had Professor McCoy sufficiently taken these and other reasons 

 into account, I am much iaclined to believe that many of the new 

 species introduced by him would have been omitted altogether, 

 while others would merely have been noticed as seeming varieties. 

 Never having yet had an opportunity of examining the specimens on 

 which his species of Cheirolepis are foimded, it is impossible for me 

 to speak at all confidently regarding them, but, from his descrip- 

 tions, and the seemingly very accurate figures given in the work 

 referred to, as well as from having examined some other specimens 



