﻿STRUCTURE OF THE PAL/EONISCIDiE. 



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prolonged and overlapped, the articular spine less marked or totally lost ; the latter also 

 disappears, though the keel endures, on the scales of the tail-pedicle. Towards the tail 

 the scales become also more acutely rhomboidal, and on the sides of the caudal body- 

 prolongation they are very acutely lozenge-shaped. Along the middle of the back is a 

 row of azygous scales ; these for a considerable distance behind the head are of 

 small size, and placed one at the point of convergence of each pair of right and 

 left dorso-ventral scale bands. In most genera, as the dorsal fin is approached, 

 these median scales become larger and imbricate more deeply, becoming first oval, then 

 acutely pointed posteriorly, while the last seems to rise up a little on the front of the 

 origin of the dorsal fin. In like manner behind the dorsal fin the median scales soon 

 become large, oval, and imbricating ; and as they proceed backwards on the dorsal 

 margin of the caudal body-prolongation, they become very acutely pointed posteriorly, 

 and anteriorly deeply notched so as to assume the form of a V. These V scales are the 

 so-called fulcra of the upper lobe of the tail, and are prominent throughout the entire 

 family j in most cases they seem to be simple (monostichous), and correspond closely with 

 the similar structures seen in Polyodon (PI. VII, fig. 3) and Acipenser. In C/ieirolepis, 

 however, they seem to become double by the splitting at the apex of the two legs of the V. 

 A few large pointed and imbricating median scales are also seen in front of the origin of 

 the lower lobe of the caudal. 



Between the ventral and anal fins a peculiar arrangement of scales is seen in many 

 genera apparently in connection with the orifice of the vent. In Elonichthys, for 

 instance, we find behind the ventrals a large median lozenge-shaped plate or shield, 

 flexed upwards to either side along its median line. Behind this, and overlapping its 

 posterior extremity, there is on each side a right and left lateral plate of an oval form, not 

 quite so large as the median one, and between these two the anal orifice was evidently 

 placed. 



Lateral Line System. — The system of "slime-canals" was doubtless well-developed 

 on the head, though only the right and left main branches can be distinctly traced for 

 any distance. These are shown in JYeviatojjfyc/nus (PI. I, fig. 7) traversing longi- 

 tudinally the parietal, frontal, and anterior frontal bones, and also less distinctly in 

 Palaoniscus (PI. I, fig. 3). Posteriorly the lateral canal passes obliquely through the 

 upper part of the supra-clavicular (PI. I, fig. 5), then enters the series of scales of the 

 lateral line, each of which it tunnels through, manifesting its presence externally by a 

 slight longitudinal elevation, on which from time to time a small pore, like a little 

 vertical slit, may be observed. The lateral line proceeds backwards to the base of the 

 caudal fin, near its bifurcation, and does not, as has been correctly pointed out by 

 Martin (op. cit.), pass along the middle of the caudal body-prolongation, as in the 

 restored figures of Agassiz and Liitken. Whether or not it proceeds further along 

 the lower margin of that prolongation towards the extremity of the upper caudal lobe, 

 as it does in Polyodon, is not clearly shown in any specimen I have seen. 



