Fossil in Glacial deposits of Scotland, Mus. Lyell; Red Crag (Walton, Essex), Coralline Crag (Sutton), Mus. S. V. Wood.
I have given so full a description of the genus that little remains to be said under the species. Generally the whole shell is covered (independently of the interfolding, oblique, articulating plates) by narrow, longitudinal ridges or folds; and by this character alone the ordinary variety of [V. Strömia] can be distinguished (as far as I have seen) from all the other species. The shell is white or dirty yellowish-brown. The scutum has the lower articular ridge on its tergal margin very narrow (but somewhat variable in width), appearing like a mere slight shoulder, against which the longitudinal axial ridge of the tergum abuts: it is not half as wide as the short, upper articular ridge. On the under side there is a very slight depression for the adductor scutorum muscle. There is considerable variation in the degree to which the transverse ledge on the under side of the fixed tergum projects, and therefore in the depth of the hollow thus formed. The specimens with the right-side, and those with the left-side opercular valves moveable, are apparently about equally numerous.
The specimen in the British Museum, from the Red Sea, was attached to a Gorgonia, and was in the same box with a [Pyrgoma]—circumstances favouring the correctness of the locality—but I am much surprised from the general distribution of the species, that [V. Strömia] should occur in so distant and isolated an area. After careful examination, I can discover no constant difference between the Red Sea and British specimens.
The specimens from the Crag have not their moveable opercular valves, which offer much more important diagnostic characters than the shell; but as far as the latter is concerned, no difference whatever can be perceived from [V. Strömia].
2. [VERRUCA] LÆVIGATA. Pl. [21], fig. [3 a], [3 b].
VERRUCA LÆVIGATA. G. B. Sowerby. Genera of Recent and Fossil Shells, Plate.
Moveable scutum, with the lower articular ridge broader than the short upper articular ridge; moveable tergum broader than high, with the upper articular ridge produced into a point.
Hab.—Tierra del Fuego; Eastern Patagonia, nineteen fathoms; Chile; Peru; Mus. Brit., Cuming, Stutchbury, Darwin: attached to shells, and often to [Balanus lævis] and [psittacus].
I can point out no difference in the shell between this species and [V. Strömia], excepting that its walls seem invariably to be smooth, which is rarely the case with [V. Strömia]; perhaps also the oblique interfolding articular plates between the several compartments are here more prominent. It appears that specimens with the left side uppermost, and therefore with the left opercular valves moveable, are considerably more common than those with the right valves moveable. The moveable scutum and tergum are articulated together by much more prominent articular ridges than in [V. Strömia], and the two valves together are broader in proportion to their height,—the height being measured from the apex to the basal margin. In the scutum the lower articular ridge is considerably broader than the short upper ridge. In the tergum, the basi-carinal corner is more rectangular, and the whole valve is nearly square: owing to the deep furrow receiving the lower articular ridge of the scutum, the axial ridge of the tergum is proportionally narrower but more prominent than in [V. Strömia]; the uppermost ridge (formed by the occludent margin of the valve) projects, especially when viewed on the under side (fig. [3 b]), as a moderately sharp point.