﻿6 GANOID FISHES OF THE CARBONIFEROUS FORMATION. 



Teleostei, but again, as in the Selachii, form a "chiasma" with decussation only of a few 

 fibres. The intestine contains a spiral valve (rudimentary in Lepidosteus). The 

 branchia? have, however, as in the Teleostei, free extremities, and are contained in a gill- 

 cavity covered by an operculum (except in the doubtful group of Acanthodei). There 

 is a swimbladder (sometimes double, as in Polypterus), provided with an air-duct. The 

 internal skeleton shows a wide range of development of its hard constituents ; in some the 

 notochord is altogether persistent {Polyodon) • in some osseous hemi- vertebrae are present 

 {Pycnodus, &c.) ; in others ring- vertebrae {Meyalichthys), while in others biconcave 

 vertebrae of the usual Teleostean type are developed, and in Lepidosteus they actually 

 become opisthoccelous. The termination of the vertebral column exhibits a portion of 

 soft unsegmented notochord. The anterior margins of the fins are frequently, though 

 not always, furnished with the peculiar little scales, or supplementary raylets, known as 

 " fulcra." The skull is, as in Teleostei and most Selachii, hyostylic, thus differing from 

 the autostylic skull of the Dipnoi. As in the Dipnoi and Teleostei, the cranial roof is 

 provided with membrane bones ; as in many Physostomous Teleostei, the maxilla forms 

 part of the edge of the mouth and ordinarily bears teeth, but the vomer is duplex. The 

 skin is covered with hard scales, plates, or scutes of various forms, save in Polyodon and 

 Chondrosteus, where these structures are confined to the termination of the body axis in 

 the region of the caudal fin. These scales are ordinarily of osseous substance and 

 covered externally with a thin layer of a peculiar structureless substance known as 

 " ganoine " — popularly as " enamel," and to which that lustrous appearance is due 

 from which the order takes its name. 



The Ganoidei are thus seen to occupy a peculiarly intermediate position between the 

 Selachii, Dipnoi, and Teleostei, having some characters in common with each ; while it is 

 at the same time difficult to find a distinctive mark which will be exclusively diagnostic 

 of every member of the entire series. With the Selachii they are connected by their 

 heart and optic chiasma, though widely separated by their free gills, opercular apparatus, 

 cranial bones, and swimbladder. Here, however, we are confronted by the singular 

 extinct group of Acanthodei, which, with an absence of several important Ganoid 

 characteristics, seem to approach the Selachii in their general aspect. Though the 

 Ganoidei are separated from the Dipnoi by their hyostylic skull, a transition to the latter 

 order is afforded by those Crossopterygian forms with acutely lobate pectorals {Holopty- 

 chius, Glyptolepis) ; while, indeed, the position in the one or in the other group of such 

 genera as Phaneropleuron and Uronemus is not by any means definitely settled. Through 

 the Amiada the Ganoidei graduate into the Teleostei, a transition not sufficiently 

 recognised by Giinther in his recent proposal to unite the Selachii, Chimceroidei, Dipnoi, 

 and Ganoidei into one subclass of Palaichfhyes, a second and distinct subclass being 

 formed by the Teleostei. 1 For, indeed, many Mesozoic genera (Thrissops, Leptolepis, &c.) 



Description of Ceratodus, 'Phil. Trans.,' 1871. 



