GEOLOGICAL SURVEY IN THE UPPER SILURIAN ROCKS OF SCOTLAND. 881 



as already noted in my Bradford " Address," # certain appearances are visible which 

 are at least strongly suggestive of eyes. These appearances, observed by Mr Tait 

 himself before the specimens were referred to me, consist of two small dark rounded 

 spots (o), one at each antero-external angle of the head, and which come out still 

 more distinctly if the specimen be wetted with a little water. In fig. 5 the dark 

 spots take on a somewhat ring-shaped appearance, the centre being paler than the 

 circumference. 



On examination with a strong lens, or with the compound microscope (one inch 



Fig. 1. — Diagrammatic restoration of 

 Thelodus Scoticus, Traq., showing 

 the dorsal fin and the position of the 

 eyes. The tail is turned round so as 

 to show the caudal fin in profile. 



Fig. 2. — A similar outline of Lanarkia spinosa, 

 Traq. In this genus (L. horrida) the eyes 

 have been found, but not the dorsal fin 

 as yet. 



objective), the spaces occupied by these dark spots are seen to be covered with minute 

 scales like the rest of the skin, but which appear as if stained by a darker colour, 

 which, as already said, is intensified the moment the surface is wetted. As the same 

 phenomenon is also observed in a specimen of Lanarkia horrida, I shall return to it 

 presently in dealing with that genus and species. 



Scales. — A specimen from Logan Water in the Geological Survey Collection shows 

 beautifully the ccelolepid character of the scales, these being split or divided horizon- 

 tally so as to expose the internal pulp cavity. The external sculpture of the pointed 



* Also in the Summary of Progress of the Geological Survey of the United Kingdom, for the year 1900, p. 175. 



