ISOCARDIA. 
15 
2. Genus. — Isocardia (Klein, 1753), LamarcJc, 1799. 
Whether the Palgeozoic forms referred to this genus really belong to it seems to 
be doubtful, and in the species described below the hinge is not sufficiently 
exposed to permit comparison. Shells have been described as belonging to this 
genus from the Silurian to the present time. 
1. ISOCAEDIA CUOKEEI, n. Sp. PI. II, figS. 11, 11 tt. 
Description. — Right valve ovoid, convex, suborbicular. Umbo large, 
prominent, facing anteriorly, and situate at about the anterior third of the 
hinge-margin. Anterior margin produced, narrow, and very convex. Inferior 
margin slightly convex. Posterior margin narrow, apparently subangular. 
Hinge-margin very convex. Hinge (only partly seen) with two long, narrow, low 
lateral teeth, separated by narrow grooves. Contour of back convex, bounded on 
the postero- superior side by a sharp, curved, overhanging angle, which runs from 
the back of the umbo to the posterior point, and separates an oblique, flat, lancet- 
shaped area, so that the valves meet above at more than two right angles. Shell- 
structure massive. Interior of valve unseen. 
Size. — Length 20 mm., breadth 19 mm., depth of single valve 9 mm. 
Locality. — There is a single specimen from Wolborough in Mr. Yicary's 
Collection. 
Bemarhs. — I have not met with any described species corresponding with this 
fossil, and as the hinge is much obscured I have found it difficult to decide its 
genus. On examining the specimen in conjunction with Mr. R. Etheridge, F.R.S., 
we came to the conclusion that it most probably was a species of Isocardia, and 
in that opinion I am confirmed by the general likeness it bears to other palaeozoic 
species which have been referred to that group. 
It has been suggested that it might be a young specimen of Megalodon 
cucullatus, Sow.,^ but a comparison of it with Sowerby's and Goldfuss's figures," 
and with numerous foreign specimens of all sizes in the British Museum, proves 
that this is not the case. The present shell is much longer, its umbo is smaller 
and more oblique, its anterior side is much larger and more convex, and the 
exposed part of its hinge does not correspond. Groldfuss describes that shell as 
1 1829, Sowerby, ' Miu. Conch.,' vol. vi, p. 132, pi. dlxviii. 
2 1834.-40, Goldl'uss, 'Petref. Germ.,' vol. ii, p. 183, pi. cxxxii, figs. S a—d. 
