﻿STRUCTURE OF THE PALvEONISCID^E. 



2:3 



The Internal Skeleton of the Body. 



Owing to the great strength and density of the external scaly covering of the body, 

 it is comparatively seldom that any very good views can be had of the delicate bones of 

 the internal skeleton, which are, in point of fact, in the majority of specimens, totally 

 obscured and covered up. It is also possible that the extent to which the parts of the 

 internal skeleton were ossified differed in different genera, though we may also feel 

 pretty certain that, taking the perished cartilaginous portions into account, its type of 

 structure was identical in all. 



Vertebral Axis. — The persistence of the notochord in the entire length of the 

 vertebral axis may, I think, be accepted as characteristic of the entire family. The entire 

 absence of vertebral centra in Palcsoniscus as well as in other extinct forms (Caturus, 

 &c), in which vertebral apophyses might nevertheless be detected, was commented on 

 by Agassiz, who was at first inclined to imagine that the missing centra had been 

 destroyed by some unknown physical force. 1 Only in Pygopterus have I seen anything 

 resembling a vertebral body. In this genus, in which, partly owing to the comparative 

 thinness and smallness of the scales, the internal skeleton is more completely seen than 

 in any other, a series of nodular-looking ossifications may generally be observed, though 

 much crushed and obscured, extending backwards from the head along the abdominal 

 region of the body in the position of the vertebral axis, and which certainly do remind us 

 of vertebral centra, as which they were considered by Agassiz. That these were 

 completely developed osseous vertebras like those of Teleostei and certain Ganoids is not 

 probable from their general appearance ; it is more likely that they had only attained 

 the stage of " hemi-vertebrae " or at most of " ring-vertebrae," and that in any case the 

 notochord passed on uninterruptedly through them. They cease to be observable in the 

 caudal region, and in no other genus of Palaoniscidce have I seen any trace of similar 

 ossifications. 



Nor have I ever seen any trace of ribs in any Palaeoniscoid Fish ; their complete 

 absence, in Pygopterus at least, seems to be conclusively proved by a fine specimen 

 from Thickley, Durham, contained in the Edinburgh Museum of Science and Art. 



In that specimen of Pygopterus a series of neural arches is, however, very distinctly 

 traceable from the head backwards. In the abdominal region of the body these are 

 rather oblong in shape, slightly falcately expanded above, and inclined a little forwards 

 as well as upwards ; from each there then proceeds a slender neural spine obliquely back- 

 wards and upwards. Anteriorly the contour of these spines is slightly curved, the 

 concavity being directed forwards. The abdominal region of the vertebral column 

 terminates a little in front of the anal fin, and from this point backwards a series of 



1 ' Poissons Fossiles,' vol. ii, pt. 1, pp. 50, 51. 



