594 



FISHES OF THE OLD RED SANDSTONE. 



parts a striking analogy with C. rostratus : thus, at its anterior extremity we find again exactly 

 the same piece which I have designated as the ethmoid bone, with this difference, that here the 

 impression of this bone is cut off square on its anterior edge ; its lateral edges being straight and 

 parallel to each other, and its posterior edge advancing towards the disk of the cranium by a nar - 

 rower pedicle. The posterior border of the head is truncated as in C. Lyellii (PI. 2, fig. 3,) ; but 

 it is directed more obliquely forward, and elevated on each side so as to form a sort of hinge, as 

 we often observe on the articulated borders of the rings of certain crustaceans. These elevated 

 edges do not however join at the nape of the neck, which is depressed. On exposing this fossil to 

 the light so as to exhibit all its inequalities, a line is distinctly traceable on the centre of it, ex- 

 tending from the ethmoid bone to the most elevated part of the cranium, the surface of which is com- 

 pletely round. On the sides of the disk is seen a sinuous furrow, which is lost in front in the edge 

 of the impression, and at about the place where, in the C. rostratus, we perceive the two lateral 

 elevations. The lateral edges of the head, especially on the posterior part, are flattened in the same 

 manner as in the C. rostratus with this difference only, that in C. Lewisii they are perpendicular 

 towards the junction of the head and body, and inclined outwards in their anterior and middle parts. 

 On the left side is still seen some trace of the inferior surface of the bones of the cranium, and im- 

 mediately at its edge, and on the posterior and inferior angle of the head, there remains a fragment 

 which displays the granular structure of its substance, and which presents a perfect identity of or- 

 ganization with G. rostratus. In the C. Lewisii, however, I have not been able to trace any lateral 

 prolongation in the shape of a horn 1 . 



"The head has something very remarkable in its form : the posterior part is narrowed and more 

 arched than the middle part, which is flattened and dilated laterally ; the snout is also flat ; the disk 

 has an oval form, truncated at its two extremities." 



This species was found in the Old Red Sandstone of Whitbach by Dr. Lloyd, together with Ce- 

 phalaspis Lloydii, §c. It was named by M. Agassiz, at my request, after the Rev. T. T. Lewis. 



4. Cephalaspis Lloydii, Agass. (PL 2. f. 7 and 9.) Recherch., vol.ii.t. 16.f.9 3 10 and 1 J . 



" This species resembles the preceding in the general form of the head (the only part preserved 

 amongst the numerous specimens I have seen) except that its anterior edge is rounder, and its pos- 

 terior extremity less narrow. Of the four species this is the one, the head of which at first sight 

 least resembles that of a fish. In the specimens where the superior surface is preserved, it is diffi- 

 cult to divest oneself of the idea, that these fossils are the shells of some molluscous animal; for so 

 completely regular are the strice, that they perfectly resemble the lines of growth of the testacea. 

 On a nearer examination, however, these striae are disposed differently from those of the shells of any 

 mollusks ; for even if we were disposed to consider these disks the shells of patelliform mollusks, 

 or even of any conchifer, the disposition of the striae would at once contradict such hypotheses. In 

 reality these strise are disposed on the sides of a central line running from the front to the back of 

 the disk, and offering in the posterior part a perfect parallelism, whilst on its anterior part they 

 follow its curvature, and in no respect resemble the concentric lines, formed by the new plates of 

 shells in their growth. By comparing many broken specimens of these fossils it is evident, that 

 this striated surface is a coat of enamel, distinct from the substance which forms the solid part of 

 the disk ; and that these stria? are grooves in the superficial part of the specimen, and not the ele- 



1 Fig. 8. represents a horn of the disk of one of the heads of Cephalaspis before described. 



