260 SILURIA. [Chap. XI. 
that monograph, as well as to the fervid writings of Hugh Miller and to 
the memoirs by Egerton and Huxley, let us now, mentioning some of the 
principal types, endeavour to determine the order in which these Ichthyo- 
lites appear geologically, or in the successive layers of this group of rocks. 
It has been shown (p. 133 et seq.) that along the frontier of the Silurian 
rocks in Shropshire and Herefordshire, where a true mineral transition is 
seen to take place between the Upper Ludlow rock and the base of the 
Old Bed Sandstone, there is also a gradual passage from the fossil Pishes 
of the one to those of the other. Thus, even beneath the lowest of the 
Bone-beds of the Upper Ludlow rock we have Pteraspis, and, again, in the 
chief Bone-bed Plectrodus mirabilis and Onchus Murchisoni, associated 
with Pterygoti and many Shells known in the beds below. In ascending 
to a higher stratum most of those mollusks disappear ; and, although the 
same Onchus is still found, we first meet with two species of the genus ; 
Cephalaspis added to Pteraspis in those strata which begin to assume the 
lithological characters of the Old Eed Sandstone. In a word, the Tile- 
stones, or beds of passage, considered in a broad sense, possess at their 
base a Shell or two and a Pish- defence, with Crustacea of the Upper 
Ludlow rock, and in their upper parts, which begin to graduate into corn- 
stones, we first find the characteristic Pishes of the Old Eed Sandstone. 
It follows, therefore, that as the grey, flag-like strata which pass up into 
reddish beds may either be viewed as the termination of the Silurian or 
the commencement of the Old Eed, the genera Cephalaspis and Pteraspis 
are typical both of the uppermost Silurian and the lowest zone of the Old 
Eed or Devonian group ; in truth, as we now know that the variegated 
concretions called cornstones are traceable down to within a very few feet 
of those transition-beds, and as Cephalaspis Lyellii, PI. XXXVI. f. 1-8, - 
and two species of Pteraspis, ib. f. 9 & 10, abound in them, there can no 
longer be any doubt on this point. Por a popular acquaintance with the 
oldest recognized Ichthyolites of the Old Eed Sandstone, the reader is 
referred to the figures in Plates XXXYI. & XXXVII. 
In adopting this view, we remove one of the difficulties which was pre- 
sented to the mind of Hugh Miller, in his endeavour to determine the 
order in which the different Ichthyolites of the Old Eed Sandstone of Scot- 
land successively made their oppearance. Grouping the Caithness flagstones 
in the lower division, and unable, on the one hand, to detect a Cephalaspis 
in them, or on the other to find the Pishes of his north-eastern tracts in 
the central parts of Scotland, he was naturally induced to suggest that the 
beds with Cephalaspis * would be found to lie above the fish-beds of Cro- 
* In formerly adopting the belief that the corn- mit of the Ludlow rock. Again, it was formerly 
stones with Cephalaspis generally represented the believed that a Dipterus (a marked Caithness 
middle beds of the Old Eed Sandstone, Hugh genus) had been found in the Upper Ludlow 
Miller was quite justified; for it was then sup- rock. This was a mistake in the original 'Silurian 
posed, even by myself, that these concretions oc- System; ' for in no subsequent researches has the 
cupied the central part of the group ; whilst we smallest fragment of a Dipterus been detected in 
now know that their inferior portion actually the bone-bed of the Upper Ludlow rock, 
graduatesdownwards into the Tilestones and sum- 
