RAMBLES OP A GEOLOGIST. 415 



sole locality in which shells had been detected in the Old Red 

 Sandstone of Scotland. But these have since been found 

 in the neighbourhood of Thurso, by Mr Robert Dick, asso- 

 ciated with bones and plates of the Asterolepis, and by Mr 

 William Watt on the opposite side of the Mainland of Ork- 

 ney, at Marwick Head. So far as has yet been ascertained, 

 they are all of one species, and more nearly resemble a small 

 Cyclas than any other shell. They are, however, more deeply 

 sulcated in concentric lines, drawn, as if by a pair of compasses, 

 from the umbone, and somewhat resembling those of the ge- 

 nus Astarte, than any species of Cyclas with which I am ac- 

 quainted. In all the specimens I have yet seen, it appears 

 to be rather a thick dark epidermis that survives, than the 

 shell which it covered ; nay, it seems not impossible that to its 

 thick epidermis, originally an essentially different substance 

 from that which composed the calcareous case, the shell may 

 have owed its preservation as a fossil ; while other shells, 

 its contemporaries, from the circumstance of their having 

 been unfurnished with any such covering, may have failed 

 to leave any trace of their existence behind them. It seems 

 at least difficult to conceive of a sea inhabited by many genera 

 of fishes, each divided into several species, and yet furnished 

 with but one species of shelL I found the quarry of Picko- 

 quoy, a deep excavation only a few yards beyond the high- 

 water mark, and some two or three yards under the high- 

 water level, deserted by the quarrymen, and filled to the 

 brim by the overflowing of a small stream. I succeeded, how- 

 ever, in detecting its shells in situ. They seem restricted 

 chiefly to a single stratum, scarcely half an inch in thickness, 

 and lie, not thinly scattered over the platform which they oc- 

 cupy, but impinging on each other, like all the gregarious shells, 

 in thickly-set groupes and clusters. There occur among them 

 occasional scales of Dipteri ; and on some of the fragments 

 of rock long exposed around the quarry-mouth to the weather 



