In the Free Church pulpit I recognized an old friend, to whom I introduced myself at the close of the service, and by whom I was introduced, in turn, to several intelligent members of his session, to whose kindness I owed, on the following day, introductions to some of the less accessible curiosities of the place. I rose betimes on the morning of Monday, that I might have leisure enough before me to see them all, and broke my first ground in Orkney as a geologist in a quarry a few hundred yards to the south and east of the town. It is strange enough how frequently the explorer in the Old Red finds himself restricted in a locality to well nigh a single organism,--an effect, probably, of some gregarious instinct in the ancient fishes of this formation, similar to that which characterizes so many of the fishes of the present time, or of some peculiarity in their constitution, which made each choose for itself a peculiar habitat. In this quarry, though abounding in broken remains, I found scarce a single fragment which did not belong to an exceedingly minute species of Coccosteus, of which my first specimen had been sent me a few years before by Mr. Robert Dick, from the neighborhood of Thurso, and which I at that time, judging from its general proportions, had set down as the young of the _Coccosteus cuspidatus_. Its apparent gregariousness, too, quite as marked at Thurso as in this quarry, had assisted, on the strength of an obvious enough analogy, in leading to the conclusion. There are several species of the existing fish, well known on our coasts, that, though solitary when fully grown, are gregarious when young. The coal-fish, which as the sillock of a few inches in length congregates by thousands, but as the colum-saw of from two and a half to three feet is a solitary fish, forms a familiar instance; and I had inferred that the Coccosteus, found solitary, in most instances, when at its full size, had, like the coal-fish, congregated in shoals when in a state of immaturity. But a more careful examination of the specimens leads me to conclude that this minute gregarious Coccosteus, so abundant in this locality that its fragments thickly speckle the strata for hundreds of yards together--(in one instance I found the dorsal plates of four individuals crowded into a piece of flag barely six inches square)--was in reality a distinct species. Though not more than one-fourth the size, measured linearly, of the _Coccosteus decipiens_, its plates exhibit as many of those lines of increment which gave to the occipital buckler of the creature its tortoise-like appearance, and through which plates of the buckler species were at first mistaken for those of a Chelonian, as are exhibited by plates of the larger kinds, with an area ten times as great; its tubercles, too, some of them of microscopic size, are as numerous;--evidences, I think,--when we take into account that in the bulkier species the lines and tubercles increased in number with the growth of the plates, and that, once formed, they seem never to have been affected by the subsequent enlargement of the creature,--that this ichthyolite was not an _immature_, but really a _miniature_ Coccosteus. We may see on the plates of the full-grown Coccosteus, as on the shells of bivalves, such as _Cardium echinatum_, or on those of spiral univalves, such as _Buccinum undatum_, the diminutive markings which they bore when the creature was young; and on the plates of this species we may detect a regular gradation of tubercles from the microscopic to the minute, as we may see on the plates of the larger kinds a regular gradation from the minute to the fall-sized. The average length of the dwarf Coccosteus of Thurso and Kirkwall, taken from the snout to the pointed termination of the dorsal plate, ranges from one and a-half to two inches; its entire length from head to tail probably from three to four. It was from one of Mr. Dick's specimens of this species that I first determined the true position of the eyes of the Coccosteus,--a position which some of my lately-found ichthyolites conclusively demonstrate, and which Agassiz, in his restoration, deceived by ill-preserved specimens, has fixed at a point considerably more lateral and posterior, and where eyes would have been of greatly less use to the animal. About a field's breadth below this quarry of the _Coccosteus minor_,--if I may take the liberty of extemporizing a name, until such time as some person better qualified furnishes the creature with a more characteristic one,--there are the remains, consisting of fosse and rampart, with a single cannon lying red and honeycombed amid the ruins, of one of Cromwell's forts, built to protect the town against the assaults of an enemy from the sea.