﻿STRUCTURE OF THE PAL^ONISCID^E. 



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rays, except a few very short and delicate ones at the extreme end of the upper lobe, 

 arising entirely from the lower aspect of the scaled caudal prolongation of the body-axis, 

 which proceeds along its upper margin. The ventrals are placed about half way between 

 the front of the head and the commencement of the caudal ; their base is in most cases 

 short, though remarkably long in Cheirolepis, Cosmoptychius, and Oxyynathus. The dorsal 

 fin never occurs in front of the ventrals, being very commonly placed more or less opposite 

 the space between those and the anal (Palaoniscus, ffloniclitliys, Bhabdolepis, Amblypterus) ; 

 in some cases its anterior rays commence only a little in front of the origin of the anal 

 (Pyyopterus, Nematopfychius, Bhadinichthys) ; in Cycloptychius it is placed quite opposite 

 the anal, while in Cheirolepis its anterior commencement is actually behind that of the 

 last-named fin. 



The rays of all the fins are very numerous, and exceed in number the ossicles sup- 

 porting them, which they also slightly overlap with their pointed proximal extremities. 

 They dichotomise freely, the division being in most cases carried to an extreme degree of 

 fineness. This process commences in the principal rays of each fin, usually towards their 

 extremities ; but in the more posterior and delicate rays, and in those composing the 

 whole upper lobe of the caudal, it commences in most cases earlier. 



In most of the genera the outer surface of the rays, save at their extreme terminations, 

 is ganoid, in some cases smooth and brilliant, in others marvellously ornamented 

 with delicate ridges and strias, as in Acrolepis, Elonichthys, &c. In Pyyopterus 

 it seems to me, however, probable that the outer surface of the fin-rays was, 

 as in the recent Polyodon, non-ganoid, and covered in the living state by a delicate 

 soft skin. 



The fin-rays are very closely set throughout, and in the anterior part of each fin the 

 demi-rays of each side overlap or imbricate over each other from before backwards ; 

 this usually ceases to be apparent posteriorly, where the rays are smaller and more delicate, 

 and, from their origin, set side by side. All the rays, also, except in the case of the 

 pectoral, are as far down as their emergence from the scales of the body divided by trans- 

 verse articulations, whose degree of closeness, usually pretty considerable, varies in 

 different genera and species. As regards the pectoral, in most cases the first ray or the 

 first two or three rays are unarticulated for some distance after their origin, the immense 

 majority of the rays being, as in the other fins, closely articulated throughout (Palao- 

 niscits, Amblypterus, Nematopfychius, &c). But in a few forms (Pyyopjterus, Oxygnathus) 

 the principal rays of the fin are not articulated at all till towards their terminations, when 

 bifurcation also sets in. 



The tendency of the demi-rays to imbrication, together with the closeness of the 

 transverse articulations, has often given rise to the idea that the fins, in certain forms at 

 least, were covered with scales. If, for the sake of example, we take such a form, as 

 Acrolepis, we find that each joint does, in fact, much resemble a scale. Exter- 

 nally it is divided by a vertical line, in front of which there is a broad surface 



