﻿INTRODUCTION. 



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specimens imbedded in rocks of different or even only slightly different mineral 

 character. While, for instance, the fishes in the hard clay-ironstone nodules of Wardie 

 afford much excellent information as to their cranial osteology, the external sculpture of 

 the scales is usually invisible, owing to the flaking off and adherence to the counterpart 

 of the external ganoine layer j on the other hand, the specimens found in the blackband 

 ironstone of Gilmerton or in the Limestone of Burdiehouse almost invariably have the 

 head in a state of complete " mash," while the outer surface of the scales is well displayed. 

 Another circumstance, which renders necessary the greatest caution, as well as the 

 examination and comparison of a great number of specimens, in the determination of 

 species, is constituted by the remarkable distortions and alterations of form which are so 

 often met with, the result of changes which have occurred after the death of the fish, and 

 before or during its entombment in the ancient mud, now hardened into the stony 

 matrix of the fossil. As a predisposing cause of these effects the soft or notochordal 

 nature of the vertebral axis in so many genera and species must have operated very 

 largely. But no one can devote close attention to the study of Palaeozoic Fishes without 

 being struck by the fact that in very many cases the attempt to define species by the 

 proportional measurements of the specimens, as they lie before us on the stone, would 

 only result in our making nearly as many species as specimens ! It is therefore in the 

 highest degree unsafe to rely on such measurements as specific characters except in cases 

 where the arrangement of the scales and of the bones of the head is clearly undisturbed, 

 and where the specimens otherwise show no evidence of lengthening out or of crumpling 

 together. 



In classifying the Fishes of the Carboniferous formation, I shall follow Professor 

 Huxley in retaining the great divisions of Fishes in general, and the conception of a 

 Ganoid Fish in particular, laid down by Johannes Muller. The class Pisces will 

 accordingly be divisible into the following seven " orders :" 



1 . Pharynyobranchii — Amphioxus. 



2. Marsipobranchii — Petromyzon, Myxine, Bdettostoma. 



3. Selachii — Scyttium, Squatina, Baia, Cestracion, &c. 



4. Chimaroidei — Chimcera, Callorhynchus. 



5. Dipnoi — Ceratodus, Zepidosiren. 



6. Ganoidei — Acipenser, Polypterus, Lepidosteus, &c. 



7. Teleostei — Salmo, Gadus, Labrus, Perca, &c. 



The Fishes whose remains are found in Carboniferous rocks are referable to the 

 orders Selachii, Dipnoi, and Ganoidei, the present Monograph dealing only with the last. 



In the Ganoidei the heart is, as in the Selachii, provided with a peculiar " conus 

 arteriosus," from which the branchial arterial trunk takes its origin. This conus 

 arteriosus is not the same as the " bulbus " or dilated origin of the branchial artery of 

 Teleostei, but is a differentiation of the anterior part of the ventricle ; it is muscular, and 

 contains internally many rows of valves. The optic nerves do not simply cross as in the 



