Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/174

84 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Dec. 8, margined by a raised edge, tinder which are placed the muscular (adductor) elevations or scars, which are longitudinally separated by a narrow concave space. Length 11, breadth 14 lines.

The internal cast of the dorsal valve was, in 1863, described and figured by Mr. Salter, the exterior and interior of the other valve being unknown to him at the time; subsequently, however, impressions of the exterior, as well as the internal cast of the ventral valve, were found by Mr. Vicary, so that it is now possible to draw up a complete description of the species. The shell described by Mr. Salter as Porambonites, sp.? (from the imperfect internal cast of a dorsal valve), appears to me to be a large internal cast of the shell under description, in which the muscular impressions are imperfectly defined; but I make this suggestion with reserve. The figure given by Mr. Salter is likewise not entirely correct. In the original specimen the cardinal angles are more rounded; and the muscular impressions, &c, are less sharply defined on the cast than they are in the figure; nor does it, or Mr. Sharpe's Portuguese specimens, to which it is assimilated, belong to the genus Porambonites.

I do not know whether it is a Silurian or a Devonian species, as no other known form has been found associated with it in the same pebble.

27. Orthis Vicaryi, n. sp. Pl. V. figs. 20-22.

Shell circular or elongated oval, slightly wider anteriorly; hinge-line straight, about half the breadth of the shell. Ventral valve moderately convex, beak slightly incurved, area narrow and small, divided in the middle by a rather wide fissure. Dorsal valve rather less convex than the opposite one. Surface of both valves marked by numerous fine thread-like radiating lines or raised striae, which increase in number by means of numerous interpolations. In the interior of the ventral valve two large, deep, oval-shaped muscular depressions occupy fully two-thirds of the length of the valve, and are divided in the middle by a narrow, angular, slightly projecting ridge. In the interior of the dorsal valve a small, bilobed cardinal process is situated between the prominent, diverging, slightly curved brachial processes; while a rather wide, rounded mesial ridge extends from under the cardinal process to about half the length of the valve, and separates into two parts the muscular (adductor) impressions. Two specimens measured:—

Length 7, width 7 lines. "  5    "   4

Of this species several examples have been found by Messrs. Vicary and Valpy. It occurs most frequently in the shape of internal casts; but impressions of the external surface are occasionally met with. It is remarkable on account of the large proportions of the elongated, oval-shaped muscular depressions in the ventral valve.

Along with this species is found a small Rhynchonella?, which, if R. ovalis, would perhaps indicate that the Orthis under description is Silurian; but the question of age will require to remain for the present undetermined.