'331 FOSSILS FROM THE 



must read by the special signification attached to them, or t< 

 somewhat resembling signs in the existing scene of things. 

 He is necessitated to interpret his ancient characters by a mo- 

 dern key, whose resolutions are but approximations ; and thus 

 his knowledge regarding them must be a knowledge consider- 

 ably mingled with uncertainty. With, however, the ganoidal 

 order of fishes the case is essentially different From the 

 peculiar armature of solid bone in which they were inclosed, 

 many of them continue to exist, not as mere teeth, spines, and 

 points, but as fishes ; they are things, not signs ; we may ac- 

 quaint ourselves as completely with their external forms, and 

 even much of their internal structure, as with the forms and 

 structure of the fishes which still exist ; and know absolutely, 

 from their study, under what peculiarities, and associated with 

 what varieties of mechanism, vertebral life existed in the ear- 

 lier periods of the world's history. 



Permit me at this stage to illustrate this special point to 

 the Association by two sets of specimens, the exhibition of 

 which, as some of them are new to Scotland, and some of them 

 absolutely so to Geology, will at the same time carry on the 

 proper work purposed in my paper. In these fragments of 

 Caithness Flagstone there are exhibited the signs, if I may 

 so express myself, of a placoid of the Old Red Sandstone. 

 They exist as minute toothed ichthyodorulites, — those of the 

 HomocantJius arcuatus, — which, though found in the Old 

 Red Sandstone of Russia, and figured by Agassiz, have only 

 veiy recently been detected in Scotland. I owe them to my 

 friend Mr Robert Dick of Thurso, to whom I am also in- 

 debted for this other ichthyodorulite of larger size [Spec. 2], 

 whicb bears a considerable resemblance to that of the Hop- 

 lacanthus marginalis, another Russian placoid of the Old 

 Red, though it may possibly belong to some undescribed spe- 

 cies of acanth. In the spine of the posterior dorsal of 

 Spinax acanthus, — the common dog-fish of our coasts, — we 



