Till very lately, it was held that the Old Red Sandstone of Scotland contained no mollusca. It seemed difficult, however, to imagine a sea abounding in fish, and yet devoid of shells. In all my explorations, therefore, I had an eye to the discovery of the latter, and on two several occasions I disinterred what I supposed might have formed portions of a cardium or terebratula. On applying the glass, however, the punctulated character of the surface showed that the supposed shells were but parts of the concave helmet-like plate that covered the snout of the _Osteolepis_. In the ichthyolite beds of Cromarty and Ross, of Moray, Banff, Perth, Forfar, Fife, and Berwickshire, not a single shell has yet been found; but there have been discovered of late, in the upper beds of the Lower Old Red Sandstone in Orkney, the remains of a small, delicate bivalve, not yet described or figured, but which very much resembles a _Venus_. (See Plate V., fig. 7.) In the Tilestones of England, so carefully described by Mr. Murchison in his _Silurian System_, shells are very abundant; and the fact may now be regarded as established, that the Tilestones of England belong to a deposit contemporaneous with the ichthyolite beds of Caithness and Cromarty. They occupy the same place low in the base of the Old Red; and there is at least one ichthyolite common to both,[AB] and which does not occur in the superior strata of the system in either country--the _Dipterus macrolepidotus_. The evidence that the fish and shells lived in the same period, and represent, therefore, the same formation, may be summed up in a single sentence. We learn from the Geology of Caithness that this species of _Dipterus_ was unquestionably contemporary with all the other ichthyolites described;--we learn from the Geology of Herefordshire that the shells were as unquestionably contemporary with it.[AC] These--the shells--are of a singularly mixed character, regarded as a group, uniting, says Mr. Murchison, forms at one time deemed characteristic of the more modern formations,--of the latter secondary, and even tertiary periods,--with forms the most ancient, and which characterize the molluscous remains of the transition rocks. Turbinated shells and bivalves of well nigh the recent type may be found lying side by side with chambered Orthoceratites and Terebratula.[AD] [Footnote AB: _Silurian System_, part ii. p. 599.] [Footnote AC: In Russia, too, as shown by the recent discoveries of Murchison, the Old Red fishes of Caithness, and the Old Red shells of Devonshire, may be found lying embedded in the same strata.] [Footnote AD: _Silurian System_, part i. p. 183.]