﻿STRUCTURE OF THE PAL^ONISCIDiE. 



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though in form and position it does bear some considerable resemblance to the large supra- 

 temporal of each side in such ganoids as Amia, Lepidotus, and Semionotus. It is succeeded 

 below by a somewhat elongated plate (*. cl.) descending obliquely downwards and back- 

 wards along the upper part of the posterior margin of the branchial slit, parallel to and 

 generally partially concealed by the hinder margin of the operculum. It is considerably 

 broader above than below ; the upper margin, horizontal when the bone is in situ, is 

 oblique to the anterior and posterior margins ; the lower extremity is rounded or obtusely 

 pointed. The anterior margin is gently sigmoid, the posterior one shows a slight inden- 

 tation above the middle, below which the bone becomes rather suddenly narrower, this 

 indentation marking the place where the lateral slime-canal enters from the first scale 

 of the lateral line to tunnel its way obliquely through the plate on its way to the cranial 

 shield. The greater part of the outer surface is ganoid and sculptured, usually with 

 delicate ridges and furrows. Now, this bone obviously corresponds with the plate 

 connecting the clavicle with the posterior part of the cranial shield in Acipenser and 

 Polyodon, lying also in precisely the same relation to the edge of the branchial slit ; that 

 plate in Acipenser being denominated "post-tempo?^" by Parker in his treatise on the 

 ' Shoulder- Girdle and Sternum.' But equally obviously does it correspond to the supra- 

 clavicular {scapular, Owen) of Polypterus and of Osseous Fishes. I have therefore 

 lettered it as supra-clavicular, being inclined to think that Parker is mistaken in his 

 determination of the corresponding plate in the Sturgeon as post-temporal, and deeming 

 it more probable that the post-temporal is absent in that genus and in Polyodon. The 

 supra-clavicular overlaps below the upper extremity of the clavicle {cl.), a bone about 

 whose homologies there is no room for dispute. This is a very prominent and well- 

 marked bone, which may be described as consisting of two parts, upper and lower. The 

 upper part, somewhat lanceolate in shape and nearly vertical in direction, sloping only a 

 very little downwards and forwards, is slightly folded, so that its outer surface comes 

 to be divided by a nearly perpendicular line into two moieties, the anterior of which looks 

 rather forwards into the branchial cavity, while the other, posterior, looks nearly straight 

 outwards. The lower portion is short and quadrate ; it is placed at a considerable angle 

 to the upper part, being reflected inwards on the ventral aspect towards its fellow of the 

 opposite side ; between the two portions posteriorly there is a well-marked notch, from 

 which the pectoral fin issues. Connected with the clavicle are two additional pieces. 

 The first of them {p. cl.) is found just behind the upper extremity of the clavicle at its 

 junction with the supra-clavicular; it is a small flat plate with nearly straight anterior 

 margin, convex posteriorly, and pointed above and below. This is the equivalent of the 

 similarly placed ossicle in Acipenser and Polyodon, which is there considered as " supra- 

 clavicular " by Parker ; the very same bone, however, as occurring in Polypterus and 

 Calamoichthys is denominated by him post-clavicular, and as such I have marked it in 

 Palaoniscus, in accordance with the views above expressed as to the supra-clavicular in 

 the Acipenseroids. The other plate {i. cl.) in special connection with the clavicle is 

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