FISHES OF THE OLD RED SANDSTONE. 



591 



compact point, which extends further back from the occipital crest (PL 2. f. 3.), and which is more 

 detached from the body in consequence of the slope of the posterior border of the disk becoming 

 wider as it recedes from the sides of the fish. The difference of form observable in PI. 2. f. 1 and 2. 

 and PI. 1. f. 1 and 2. appears to arise solely from the different states of preservation of the specimens, 

 and chiefly that in PI. 1. f. 1. and PI. 1 and 2. f. 1. the bones were as completely extended as their 

 articulation would admit of, at the time when they were surrounded by the substance which formed 

 the rock in which they are imbedded j whilst in the other specimens they were more pressed toge- 

 ther, the pressure giving to them a narrower form. 



" It is very probable that the heads of these fishes being so frequently found detached from the 

 bodies, may be accounted for by the great difference which exists in the structure of these two 

 parts, and above all in the disproportion of their dimensions and forms, which would offer a distinct 

 resistance to the pressure, to which these animals must have been exposed. If, on the other hand, 

 the heads usually present their superior surface to us, it is because their inferior surface, the cavity 

 of the mouth, the branchial arch and sinuosities of the inferior bones of the cranium, are points of 

 support comparatively more solid, and more adapted for sustaining the matter which has filtered 

 into them, than a large surface slightly convex, which would naturally be detached from the rock 

 wherever a separation was formed in it. 



" The body most resembles that of certain fishes of the family of Lepidoides, but differs consider- 

 ably from them in the two dorsal fins, in the anal being thrown further back, and in the singular 

 scales with which it is covered. Its form is that of an elongated spindle, swelling out on the 

 anterior parts, and narrowing insensibly to the end of the tail, which is proportionately very 

 slender, since its diameter does not exceed a quarter of the width of the body near the nape of the 

 neck. 



"The first dorsal is placed on the most elevated part of the back immediately behind the occipital 

 crest. Its existence is only known by the impression of its rays. On its anterior edge are two 

 grooves, somewhat larger than those which succeed ; there are certainly the impressions of two 

 large rays, the first of which may have been short, and attached the whole length of the second, 

 which extended probably to the extremity of the fin. It is impossible to decide whether there 

 were any small imbricated rays. The other rays have left no other traces of their presence, than a 

 striated appearance on the edge of this part of the back, to the middle of which they extended. 



" The second dorsal is more distinct; its anterior edge is supported by a very large ray, the trans- 

 verse articulations of which are very near each other, and on the edge of it are observable very 

 small imbricated rays, closely pressed against the larger. The rest of the fin, which appears to 

 have extended to the very slender pedicle of the tail, is only shown by a striated mark, parallel to 

 the anterior ray, of which the striae were the small blunt rays of the strong part of the fin. The 

 relative position of the two dorsals is exactly indicated in the engraving, PL 1. f. 2. We here see, 

 that behind the neck and on the posterior part of the back, the scales are not united, but have an 

 interval between, on which the rays of the fin were inserted. 



u The anal fin has not even left so distinct a mark of its presence j by comparing the two sides of 

 the specimen represented PL 1. f. 1. we merely discover that it was placed further back than the 

 second dorsal, so much so that its anterior edge corresponded to the middle of this dorsal. Its 

 position is also indicated by the interruption to be remarked on the scales of the edge of the belly. 

 The caudal fin had no large rays ; the lobes are only indicated by the particular colour of the 

 rock j the inferior one extends to the middle of the superior. Their insertion is very oblique, so 

 that the prolongation of the pedicle (ptfdicule) of the tail is proportionately very long. Its superior 



4 F 2 



