﻿1851.] MURCHISON SILURIAN ROCKS OF SCOTLAND. 151 



places, both on the coast and in the interior, a very large species of 

 Orthoceratites, together with other forms of that genus, a Cyrto- 

 ceras, and one or two species of Graptolites, and some flattened 

 Brachiopods, including Orbicula. The large species of Orthoceratite 

 being unknown to myself, and not being identifiable with any published 

 species, I referred it to my friend M. Barrande (then on a visit to 

 our country), and he recognized it to be one of the Bohemian species, 

 occurring in the lower part of his Upper Silurian division*. 



Now, in Bohemia this very species of Orthoceratite occupies the 

 same place as the greater number of species of Graptolites, i. e. the 

 schists and flags at the base of the Upper Silurian rocks ; and it is 

 further remarkable that there also it lies in schists and nodules of 

 hard impure limestone so like those of Ayrshire, that on inspecting 

 them M. Barrande declared to me he might even produce our Scottish 

 rocks as Bohemian specimens. Such is also the position of the 

 greater number of the Orthoceratites in the original Silurian region 

 of England ; and of the species which specially characterize the Upper 

 division we find here the O. virgatum (Sil. Syst.), a form to which 

 allusion will hereafter be made as occurring at Balmae Head in Kir- 

 cudbright. On the other hand, in those Scottish rocks I observed one 

 Orthoceratite with a lateral siphon which it is difficult to separate 

 from the large 0. vaginatum of Scandinavia and Russia, which I have 

 described as characteristic of the Lower Silurian f . In addition to 

 one or two other species of Orthoceratite, whose specific character 

 cannot be defined in the absence of their shelly covering, MacCallum 

 has procured from the same group a few speci- 

 mens of Cyrtoceras which much resemble forms 

 characteristic of the Upper Silurian rocks of Eng- 

 land ; with these is found (usually much flattened) 

 a thin, finely striated Orbicula, like O. striata of 

 the Ludlow rocks (see fig. 7) . The question then 

 occurs, is this zone of the same age as that of Bo- 

 hemia in which M. Barrande has collected above 

 200 species of Cephalopoda, including Orthocera- 

 tites, Phragmoceras, Cyrtoceras, Gomphoceras, 

 j icua. crassa, a . an( j a mu }titude of Graptolites ; the whole there 

 occurring, as I can indeed myself testify, in rocks undistinguish- 

 able from those now under consideration ? The Graptolites in these 

 beds, having been examined by M. Barrande and Mr. Salter, prove 

 to be a double-graptolite very like G. pristis, Hall, a species nearly 

 allied to G. colonus, Barr., and G. tenuis, Portl. Judging from this 

 group of fossils alone, I should (independently of the proofs of suc- 

 cession) be disposed to consider those orthoceratite-flags and grap- 

 tolite-schists of Piedmont, Ardwell, and Penwhapple Burn, as superior 



* Professor M c Coy has, I find, lately published this species under the name of 

 O.politum; but as this memoir is intended to be useful in leading Scotch geolo- 

 gists to be acquainted with the chief types of their oldest fossiliferous rocks, this 

 Orthoceratite is here figured, although it will probably be also given in a work 

 on the Palaeozoic Rocks now in course of preparation by Professor Sedgwick. 



t See Russia in Europe, vol. i. chap. 2. 



