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 quadrata 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, 

 5 j inches ; height, 3J inches ; convexity of the larger valve, 2j inches. 



Geological Positions and Localities. Mr. 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 immediately 

 underlying the bed of hard, cream-coloured limestone with Nerinaeas, 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 PECTINTFORMIS, Scttlotlt. Tab. XXXVI, fig. 1. Part II, 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 



