of Silurian Bocks in Cornwall, 37 



The fosssls found by Mr Peach at Gerrans Bay, as determined by 

 Mr J. Sowerby, are Orthis lata^ 0. orbicularis^ another species re- 

 sembling O.pHcata^ and a fourth, which does not appear to have been 

 pubhshed. At Caerhayos, Mr Peach has collected other forms of 

 OrthidcB, one of which approaches nearest to the 0. alternata of the 

 Silurian system. The remainder are not, however, referrible, as ho 

 had supposed, to Leptcona lata, Terebratula nucula, Atri/pa stria- 

 tula, &c. The fossils from the Great Peraver quarries in Gorran 

 Haven, on the eastern face of the Dodman, are still more decisive; 

 for the species which Mr Peach has named Orthis flabellulum and 

 0. testudinaria both belong, unquestionably, to the Orthis (callac- 

 tisB)* calUgramma (Sil. Syst.), and with it is a form undistinguish- 

 ablo from the Orthis (canalis) elegantula (Sil, Syst.). The only 

 well preserved trilobite in this rock appears to me to be the Cali/- 

 mene pidchella (Dalman) ; a second species resembles C Blumeti- 

 bachii. 



No one accustomed to the Palasozoic rocks can throw his eye over 

 the fossils from these three localities, without at once recognizing 

 them as true Silurian type?. They have an entirely distinct fades 

 from the fussils of the overlying Devonian system ; none of the spe- 

 cies so abundant in North-western Cornwall are here present. With 

 my imperfect knowledge of the country, it would be premature to 

 say, that subdivisions can be established in this highly dislocated re- 

 gion, so as to define Upper and Lower Silurian bands. But it may 

 yafely be asserted, that the fossils of Gorran Haven are Lower Silu- 

 rian types ; there being no one species more characteristic of the in- 

 ferior portion of that system than the Oi'this caligramma, which, m 

 Shropshire and the adjacent Welsh counties, is found to range down- 

 wards, from the very uppermost beds of the Caradoc sandstone into 

 the heart of the Snow^don slates, and is equally typical of the Lower 

 Silurian rocks of Russia and Scandinavia. At the same time, I do 

 not think that the Gorran Haven beds lie deep in the Lower Silurian 

 group : they probably represent the upper portion only of the Cara- 

 doc sandstone ; for the Orthis canalis, or elegantula, and the Calif- 

 menc pulchella\ (Dalm.), closely allied to C. Blumenbachii are Wen- 

 lock, as well as Caradoc, fossils. Judging from the fossils only, I 

 should say that the beds at Gerrans Bay with the Orthis orbiaalaris 



* This shell was called Orthis callactis Ji in the Sil. Syst., Plate 19, fig. 5, 

 but subsequent comparisons have shewn that it is identical with the 0. calU- 

 gramma (Dalman) of Scandinavia, Russia, &c. 



In like manner, the Orthis canalis of the Silurian System has proved to be 

 the 0. elegantula of Dalman ; and the names of that author being the oldest, 

 are now necessarily adopted. 



t The Calymene pulchella (Dalm.) occurs both in the inferior part of the Up- 

 per Silurian, and the higher part of the Lower Silurian, in Sweden, Siluria, 

 and Wales. 



