THE OLD BED SANDSTONE.

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with Agassiz j and many a query had he to put to him ; andnever, surely, was inquirer more courteously entreated, orhis doubts more satisfactorily resolved. The reply to almostmy first question solved the enigma of nearly ten years’standing. And finely characteristic was that reply of thefrankness and candour of a great mind, that can afford tomake it no secret that in its onward advances on knowledgeit may know to-day what it did not know yesterday, and thatit is content to “ gain by degrees upon the darkness.” “ Hadyou asked me the question a fortnight ago,” said Agassiz, “ Icould not have replied to it. Since then, however, I haveexamined an ichthyolite of the Old Red Sandstone in whichthe vertebral joints are fortunately impressed on the stone,though the joints themselves have disappeared, and which,exactly resembling the vertebrae of the shark, must have beencartilaginous.” In a subsequent conversation the writer wasgratified by finding most of his other facts and inferencesauthenticated and confirmed by those of the naturalist. Ishall attempt introducing to the reader the peculiarities, ge-neral and specific, of the ichthyolites to which these facts andobservations mainly referred, by describing such of the fami-lies as are most abundant in the formation, and the points inwhich they either resemble or differ from the existing fishof our seas.

Of these ancient families, the Osteolepis, or bony-scale (seePlate IV., fig. 1), may be regarded as illustrative of the ge-neral type. It was one of the first-discovered of the Caithness fishes, and received its name, in the days of Cuvier ,from the osseous character of its scales, ere it was ascertainedthat it had numerous contemporaries, and that to all andeach of these the same description applied. The scales ofthe fishes of the Lower Old Red Sandstone, like the platesand detached prickles of the purely cartilaginous fishes, werecomposed of a bony, not of a horny substance, and were all