THE OLD RED SANDSTONE. 155 
tychius. ‘The large scales and plates, and the huge teeth, 
belong to this genus. It was first introduced to the notice 
of geologists ina 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 hesita- 
tion, to the genus Gyrolepis ; and the doubts of both natural- 
ists 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 gen- 
tleman 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 speci- 
men 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 
amount of negative evidence may be dissipated by a single positive 
fact, and to inculcate on the geologist the necessity of cautious induc- 
tion. An individual Holoptychius of Thurso must have been at least 
thrice the size of the Holoptychius of the Upper Old Red formation, 
as exhibited in the specimen of Mr. Noble, of St. Madoes. 
16 
