Geology. 



419 



Lingula cornea, &c. These again are covered by less massive beds, 

 which contain fragments of plants and large Pterygoti, and which gradu- 

 ate upwards insensibly into more micaceous sandstones, often splitting 

 into tiles. The Lingula cornea and Trochus helicites together with species 

 of Modiolopsis and a small Beyrichia, all generally considered character- 

 istic of the uppermost Ludlow rock, prevail throughout these strata, with, 

 occasional carbonaceous matter and traces of land vegetation ; clearly 

 indicating an upward passage towards the younger formation of Old 

 Red Sandstone. The last-mentioned (or middle) fish bed is probably 

 the same as that which Sir R. I. Murchison described in the Silurian Sys- 

 tem as occurring in Clun Forest and other places. It has recently been 

 laid open by the cutting of the railroad northeast of the town of Ludlow, 

 and exhibits similar relations, — a grey rock beneath passing into an over- 

 lying micaceous sandstone. The same succession is obscurely traceable 

 on the right bank of the Feme, below Ludford. This bed is also charac- 

 terized by the presence of vegetable remains, seed vessels, jaws and spines 

 of Onchus, with fragments of crustaceans (Pterygotus and Eurypterus), 

 in short, just the same assemblage as that which occurs at Bradnor Hill, 

 near Kington, and has been described by Mr. Banks. Again, on the right 

 bank of the Tane, the next strata in ascending order which are visible, 

 and which have lately been accessible owing to the dry weather, consist 

 of micaceous brownish red sandstone and red marls, with true cornstone 

 concretions, exposed in the bed of the river, which are again followed by 

 other marls and sandstones, surmounted by a band of coarse, greenish, 

 micaceous sandstone, in which are found remains both of fishes and of 

 Pterygotus. The fish remains consist of distinct jaws and teeth of con- 

 siderable size, of fin defences (Onchus), and the heads of Cephalaspis 

 Lyellii, and a new species ; together with these, the Lingula cornea 

 occurs. The genus Pterygotus having now been found through the 

 upper Silurian rocks, and even so low down as the upper Caradoc forma- 

 tion, can no longer be considered characteristic of the transition beds be- 

 tween the Silurian and Devonian rocks ; and as the genera Cephalaspis and 

 Pteraspis are now known to extend their range into true upper Ludlow 

 strata, our views regarding the precise zoological characters, which sepa- 

 rate the two formations, must be modified accordingly. As regards the 

 English frontier of the Silurian rocks, the phenomena present no am- 

 biguity ; for all the strata, from the lowest bone-bed of the true Ludlow 

 rock, which contains so many species of shells of Silurian age, to the 

 uppermost fish-bed, which must be included in the Old Red Sandstone, 

 do not exceed forty or fifty feet in thickness, — the upper part of the series 

 constituting a true mineral and zoological passage into the Old Red Sand- 

 stone. In conclusion, the author observed, that the lithological term 

 " tilestones" if applied either to the top of the upper Ludlow rock or to the 

 base of the Old Red Sandstone exclusively, might mislead; but if ap- 

 plied generally to the beds of transition between the two deposits, it may 

 still be a convenient and applicable term. 



6. On a Fossil Mammal (Stereognalhus ooliticus) from the Stones- 

 field Slate ; by Prof. Owen, (Proc. Brit. Assoc., Aug., 1856 ; Ath., No. 

 1503.)— Prof. Owen exhibited, by favor of the Rev. J. P. B. Dennis, M. A., 

 a portion of a lower jaw, with three molar teeth* of a small mammal* 



