BIVALVIA. 39 
illustration it is not surprising to find that in the plates to the ‘Geology of Yorkshire,’ 
and in the ‘Petrefacta’ of Goldfuss, two very different species of Perna (flattened, 
equivalve, and rugose) were figured for the Perna guadrata of Sowerby. 
The convexity of the left valve, little remarkable in young specimens, becomes very 
considerable with advance of growth; the test upon the anterior side is moderately thick, 
but the posterior side is delicate and is rarely preserved entire. Upon the smaller of the 
specimens figured the portion denuded of the test exhibits obscure, concentric, and 
radiating striations in the convex valve; the same feature is also visible upon the surface 
of the cast of the smaller valve figured by Mr. Sowerby; it must therefore have existed 
upon the inner surface of the very thin, nacreous layer of the test, which has not been 
preserved ; the exterior surface of the test is quite destitute of ornamentation. 
Dimensions. Length of our largest specimen, in the direction of the hinge-line, 
53 inches; height, 32 inches ; convexity of the larger valve, 24 inches. 
Geological Positions and Localities. My. Sowerby’s specimen was obtained in the 
Cornbrash at Bulwick, Northamptonshire, and, as far as can be ascertained, no second 
example has been obtained from that locality. In the Inferior Oolite of the vicinity of 
Nailsworth the present author has procured specimens at several quarries, in a single 
bed; its position being the highest bed of the white building-freestone, and imruediately 
underlying the bed of hard, cream-coloured limestone with Nerinzas, which appears to be 
special to the Nailsworth valley. Perna quadrata does not appear to be very uncommon ; 
but owing to the thinness of the fibrous test, it can only be disengaged from the Oolite by 
a tedious and difficult process; more frequently, however, the shell is found to have been 
crushed or imperfectly preserved at its posterior side. 
Lima pectintrormis, Sch/oth. Tab. XXXVI, fig. 1. Part IT, Tab. VI, fig. 9. ' 
In figuring a larger and more characteristic example of this shell some additional 
remarks may be allowed. It is widely diffused, abundant and of large dimensions in the 
upper portion of the Inferior Oolite, rare and delicate in the Great Oolite, rare in the 
Cornbrash, in the Kelloway Rock and Oxford Clay ; it reappears in considerable numbers 
in the Coralline Oolite, assuming all its pristine varieties of form; these are sufficiently 
remarkable. In its young condition it was gregarious, and probably was attached by one 
of the valves to the ground; such, at least, seems an easy explanation of the fact that the 
upper surface of a slab of stone covered with the species usually discloses only the inner 
surfaces of single valves, the other valves having probably been removed by marine action 
in their dead state; but although young and thin, the specimens in this condition often 
attained to the full dimensions of the species, the radiating flutings of the external surface 
being almost equally strongly marked upon the inner surfaces, in which state, also, the 
muscular scar is not distinguishable, and when the valves are closed the umbones touch 
each other. In old specimens, owing to a continual deposition of shell upon the inner 
