NOTES. 303 



scone, there is also a gradual passage from the fossil fishes of 

 the one to those of the other. Thus, even beneath the lowest 

 of the bone-beds of the Upper Ludlow rock we have the 

 Pteraspis, and again in the bone-bed, the Plectrodus Mira- 

 bilis and Onchus Murchisoni, associated with Pterygoti, and 

 also with many shells known in the inferior layers. In ascend- 

 ing to a higher stratum, most of those molluscs disappear ; 

 and, although the same Onchus is still found, we first meet 

 with two species of the genus Cephalaspis, added to the Pter- 

 aspis, in those strata which begin to assume the lithological 

 characters of the Old Red Sandstone. In a word, the tile- 

 stones or beds of passage, considered in a broad sense, inclose 

 at their base a shell or two and a fish-defence, with Crustacea 

 of the Upper Ludlow rock ; and in their upper parts, which 

 begin to graduate into cornstones, we first find the charac- 

 teristic fishes of the Old Red Sandstone. It follows, there- 

 fore, that as the gray fiag-like strata which pass up into 

 reddish beds may either be viewed as the termination of the 

 Silurian or the commencement of the Old Red, the genera 

 Cephalaspis and Pteraspis are typical both of the upper- 

 most Silurian and the lowest zone of the Old Red or De- 

 vonian group. In truth, as we now know, the varied con- 

 cretions called cornstones are traceable down to within a very 

 few feet of those transition beds ; and as the Cephalaspis 

 Lyelli and two species of Pteraspis abound in them, there 

 can no longer be any doubt on this point. 



" In adopting this view, we remove one of the difficulties 

 which was presented to the mind of Hugh Miller, in his en- 

 deavour to determine the order in which the difierent ichthyo- 

 lites of the Old Red Sandstone of Scotland successively made 

 their appearance. Grouping the Caithness flagstones in the 

 lower division, and unable, on the one hand, to detect a Ce- 

 phalaspis in them, or, on the other, to find the fishes of his 

 north-eastern tracts in the central parts of Scotland, he wa.s 



