﻿26 



GANOID FISHES OF THE CARBONIFEROUS FORMATION. 



affixed to the front of its lower extremity, and passes horizontally forwards on the 

 " isthmus." It is acutely triangular in shape, with the apex directed forwards, and is 

 sharply folded longitudinally in a direction continuing forwards the line of flexure 

 between the upper and lower parts of the clavicle, consequently the outer surface of this 

 plate is divided into two portions, one of which looks upwards and a little outwards into 

 the branchial cavity, while the other is seen on the ventral surface of the isthmus, articu- 

 lating in the mesial line with its fellow of the opposite side, and ordinarily more or less 

 overlapped and concealed by the branchiostegal rays. This is the plate which in 

 Cheirolepis was mistaken by Powrie for a principal "jugular.'' It is, however, most 

 clearly the equivalent of the infra-clavicular plate of Gegenbaur {inter-clavicular, Parker) 

 occurring in the recent Acipenseroid and Polypteroid fishes in a similar position in relation 

 to the clavicle and to the isthmus. 



In all the genera of T 'alceoniscida the above-described bones of the shoulder-girdle 

 agree almost perfectly in their respective forms. Owing to its small size, however, the 

 post-clavicular is only observable in especially favorable specimens. 



That no trace of scapular or coracoid elements should have become as yet discovered 

 is hardly a matter of astonishment, But in two specimens I have noticed what seems to 

 me to be clear evidence of ossified radials supporting the rays of the pectoral fin. 

 One is the left pectoral of an example of NematoptycJiius Greenocldi, in which 

 certain slender ossicles are seen in connection with the origin of the rays; they 

 increase in length from the lateral towards the medial edge of the fin and seem to be 

 rather constricted in the middle, expanded at the extremities. Unfortunately, the speci- 

 men is not in very good preservation, so that the contour of these little bones is not so 

 definite as might be wished for. The other specimen is one of Cosmoptychius striatus, 

 and in it four such ossicles may be seen ; they are proportionately shorter than in the last 

 example, especially at the lateral edge, the one second removed from which is very 

 distinctly marked and is, in hourglass fashion, constricted in the middle. 



The Dermal Fin-rays. 



In the form, arrangement, and structure of their fins, the resemblance between the 

 Palceoniscida and the recent Acipenseroids is very marked, and the deviation in these 

 respects from the type of Lepidosteus equally striking. 



The fins vary considerably in size in the different genera, but not much in shape, 

 being always more or less acuminated. The dorsal and anal are usually rather triangular 

 in shape from the great length of the anterior rays and the shortness of the posterior 

 ones, the hinder sloping border being gently concave ; in Pyyoptents the posterior short- 

 rayed part of the anal is produced backwards for a little distance after the manner of a 

 fringe. The caudal fin is always deeply bifurcate, and more or less inequilobate, the 



