﻿32 GANOID FISHES OF THE CARBONIFEROUS FORMATION. 



Dentition. 



Teeth occur in all the genera on the dentary bone of the mandible, on the maxilla, 

 and, in Nematoptychius at least, they are distinctly seen on the premaxilla, so that their 

 occurrence on that little bone may be taken for granted also as regards other forms, 

 though, owing to the crushed condition of the head, the premaxilla is rarely recognisable. 

 They are frequently observable also on the splenial bone (Palaoniscus), and on the 

 palate (A/nblypterus), though here again, for obvious reasons, it is impossible in the 

 meanwhile to obtain accurate information regarding all the members of the family. 



The teeth are in almost every instance slender, conical in shape, round in transverse 

 section, acutely pointed, more or less incurved towards the apex, and furnished with a 

 glistening enamel cap which is frequently very distinctly marked off externally. Below 

 the enamel cap the surface is smooth, or may be ornamented with delicate striae, or with 

 minute longitudinal indentations ; very rarely is there any indication of plication at the 

 base. The latter occurs, however, very distinctly in Saurichthys, a genus whose reception 

 into the Palaoniscidce has been suggested by Martin, the teeth showing in other respects 

 a marked correspondence with those of Pyyopterus and Acrolepis. Microscopically the 

 teeth are distinguished by the possession of a very spacious simple pulp-cavity at the base, 

 which cavity becomes very rapidly attenuated into a narrow tube as it passes upwards 

 through the tooth ; the dentine, displaying very numerous fine tubules radiating from 

 the central cavity or canal to the periphery, is surmounted at the apex by a hollow-conical 

 cap of structureless enamel. From this enamel cap there extends downwards over the 

 whole exterior of the tooth, excepting the very base, an exceedingly thin and delicate 

 structureless film or continuation of the same substance. Such is, at least, the microscopic 

 structure of the teeth of Nematoptychius Greenockii, the only species which I have myself 

 investigated in this manner ; it is in complete accordance with the figure given by Agassiz 

 of the structure of the teeth in Pyyopterus, 1 and the description by Messrs. Hancock and 

 Atthey of those in Elonichthys Egertoni? 



The form and arrangement of the teeth have given rise to a considerable amount of 

 confusion in the classification of these fishes. By Agassiz the teeth in Palceoniscus and 

 Amblypterus were stated to be "en brosse," and this seems to have been the principal 

 reason why he disjoined these genera as "Lepidoids" from their natural allies 

 Pyyopterus and Acrolepis, which, from their possession of conspicuous laniaries, he placed 

 among the " Sauroids." 



In Palceoniscus proper (e. g., P. comptus, Freieslebeni, &c.) the teeth are very small, 

 being only with difficulty and in favorable specimens observable at all ; they are closely set 

 in the maxilla and dentary of the mandible, those more internally placed being larger than 



1 Poissons Foss., 'Atlas,' vol. ii, tab. H, fig. 1. 



2 ' Ann. and Mag. Nat. Hist.' (4), i, p. 358, pi. xvi, figs. 1 and 2. 



