BIVALVIA. 73 



Unicardium varicosum, Sow., Sp. Tab. VIII, figs. 7, la, b ; Sa, b. 



Venus vaeicosa, Sow. Min. Con., t. 296, 1819. 

 ? Unicardium corbisoideum, VOrLigny. Prod, de Paleont., i, p. 309, 1850. 

 — varicosum, D'Orbigny. Ibid., p. 310. 



Testa subglobosd, nmbonibus magnis, mediants antrorsum incurvis, lateribus brevibus, 

 postice subtruncato, marginibus rotundis, plicis concentricis tenuibus irregularibus. 



Shell very thin, subglobose ; urnbones large, mesial directed forwards ; sides of the 

 shell short, more especially the posterior side, which, differing from the usual form of the 

 genus, is somewhat shorter than the other side ; the margins of the valves are rounded 

 and slightly irregular ; the concentric plications are not prominent. 



The nuclei of this species are impressed with one or more strongly-marked grooves, 

 which pass downwards from the urnbones towards the inferior border in each valve. 

 Mr. Sowerby remarks that this species is " not remarkable for anything but the furrows 

 that occur along the middle of the specimens, all of which are casts in a light-coloured 

 limestone ; the farrows are two upon each valve, one of them much larger than the other, 

 and terminated before reaching the edge by a deep hollow ; corresponding ridges must 

 have existed inside the shell, but whether they were visible externally cannot now be 

 discovered ; the concentric furrows that are strongly marked upon some specimens would 

 seem to indicate a thin shell. It is nearly globose, but not so deep as long; the line of 

 the hinge is two thirds as long as the shell, and nearly straight ; other characters of the 

 hinge are not discoverable; the beaks are much incurved." (Min. Con., vol. iii, p. 173.) 



Localities. Casts occur in the upper marly deposits of the Oolite at Felmersham, 

 Blisworth, Kingsthorpe, Oundle, &c. The shells occur rarely in the Great Oolite of 

 Minchinhampton Common. 



Unicardium impressum. Tab. VIII, fig. da, b, c. 



Testa ovato-obliqua nut subquadratd, convcxd ; vmbonibus contiguis sitbmcdianis, de- 

 pressis, latere antico brevi, margine ejusdem rotundato, latere postico magis producto margine 

 oblique declini, margine superiori subhorizontali recto, basi curvatd, plicis concentricis 

 magnis irregularibus. 



Shell obliquely ovate, or subquadrate, convex ; urnbones submedian, contiguous, and 

 depressed ; anterior side short, its margin rounded ; posterior side more lengthened, its 

 margin sloping obliquely ; superior border nearly horizontal and straight, gaping slightly ; 

 lower border curved ; concentric plications large and irregular. 



In its young state this species is very delicate, more transverse or oblong, and depressed, 

 its surface is nearly smooth. It is only in a very advanced stage of growth that the 

 surface acquired large concentric folds, and the figure becomes subglobose, but the degree 

 of obliquity and convexity varies very much even with individuals of the same size. The 



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