Of this upper formation the most characteristic and most abundant ichthyolite, as has been already said, is the _Holoptychius_. The large scales and plates, and the huge teeth, belong to this genus. It was first introduced to the notice of geologists in a paper read before the Wernerian Society in May, 1830, by Professor Fleming, and published by him in the February of the following year, in _Cheek's Edinburgh Journal_. Only detached scales and the fragment of a tooth had as yet been found; and these he minutely described as such, without venturing to hazard a conjecture regarding the character or family of the animal to which they had belonged. They were submitted some years after to Agassiz, by whom they were referred, though not without considerable hesitation, to the genus _Gyrolepis_; and the doubts of both naturalists serve to show how very uncertain a guide mere analogy proves to even men of the first order, when brought to bear on organisms of so strange a type as the ichthyolites of the Old Red Sandstone. At this stage, however, an almost entire specimen of the creature was discovered in the sandstones of Clashbennie, by the Rev. James Noble, of St. Madoes, a gentleman who, by devoting his leisure hours to Geology, has extended the knowledge of this upper formation, and whose name has been attached by Agassiz to its characteristic fossil, now designated the _Holoptychius nobilissimus_. His specimen at once decided that the creature had been no _Gyrolepis_, but the representative of a new genus not less strangely organized, and quite as unlike the existences of the present times as any existence of all the past. So marked are the peculiarities of the _Holoptychius_, that they strike the commonest observer.