264 BRITISH SILURIAN BRACHIOPODA. 



remarkable on account of the almost equal convexity of its valves. I have counted as 

 many as 120 raised striae round the margin of each valve. 



Position and Locality. It was found for the first time by Messrs. Gray and Capewell 

 in the Wenlock Limestone and Shales during the excavation of the Rushall Canal near 

 Walsall. It was obtained, likewise, by the Rev. H. D. Day in Wenlock Shales at Build- 

 was, near Wenlock. 



Orthis alternata, Soio. PI. XXXI, figs. 1 — 8. 



Oethis alternata, J. de C. Sow. Sil. Syst, p. 638, pi. xix, fig. 6, 1839. 

 Strophomena alternata, Ptiillips and Salter. Mem. Geol. Survey, vol. ii, p. 380. 



1848. 



Orthis — and 0. retrorsistria, Salter. Mem. Geol. Survey, vol. iii, 



p. 340, pi. xix, figs. 11—13, 1866. 

 — — Salter. Siluria, 4th ed., p. 526, 1867. 



/Spec. Char. Transversely semicircular and flattened, sometimes a little longer than 

 wide ; hinge-line slightly less than the breadth. Ventral valve gently convex, area narrow, 

 fissure wide, open ; beak not projecting. Dorsal valve nearly flat, or convex only along 

 the middle, the lateral portions of the valve being slightly concave ; hinge-area narrow. 

 Surface of both valves finely radiated by numerous thread-like striae, which increase in 

 number as they approach the margin by bifurcation, as well as by the interpolation of 

 smaller striae, the whole surface being crossed also by numerous concentric lines of 

 growth. In the interior of the ventral valve the saucer-shaped muscular cavity is formed 

 of two elongated, deviating muscular depressions, with raised edges or margins. In the 

 interior of the dorsal valve there is a small cardinal process between two small, curved, 

 projecting, brachial processes, while under the cardinal process commences a median 

 ridge which extends to about half the length of the valve, and thus separates the two 

 pair of adductor impressions. Two specimens measured — 

 Length 11, width 14, depth 2 lines. 

 » 13, „ 14, „ 2 „ 

 Ohs. This Strophomena-shaped shell was in 1839 described and figured by Mr. J. 

 de C. Sowerby, and placed by him in the genus OrtJiis. In 1848 Messrs. Phillips and 

 Salter considered it to be a Strophomena closely allied to S. expansa, but in 1866 the last- 

 named palaeontologist, relinquishing that view, replaced it with Orthis, stating at the same 

 time that " the orginal figure by Sowerby in the ' Silurian System ' is from the typical 

 variety so common in the ' Jacobstones ' of Shropshire ; but the smaller and wider form 

 figured here (from an unpublished plate in the possession of the Geol. Survey) is far more 

 common in Wales, and we are able to show in it the large central tooth [cardinal process] 

 in the dorsal valve, which proves it, notwithstanding its resemblance to Strophomena, a 



