66 MOLLUSCA FROM THE GREAT OOLITE. 



Isocardia. Lam. 1799. 



Gen. Char. Cordiform, regular, ventricose ; unibones prominent, distant, diverging, 

 involute ; hinge with two compressed cardinal, and one compressed lateral tooth in each 

 valve ; ligament external, bifid, diverging in the direction of the umbones. 



Isocardia tenera, Sow. Tab. VII, fig. 1, la. 



Isocardia tenera, Sow. Min. Con., p. 494, t. 29.i, f. 2, 1821. 



— — Deshayes. Traite Elementaire de Couch., ii, p. 27, t. 24, f. 6, 1 . 



Ceromya tenera, Agassi:. Etud. Cat., t. 8 — e, f. 1 — 12, p. 34. 

 ?Var. Isocardia tumida, Phil. Geol. Yorksli., i, t. 4, f. 25, 1835. 



Testa iiucleo mhtrigono, inflato ; umboftibus mediants, altis antrorsiim incurvis ; latere 

 antico lata, striis concentricis mbtillissimis. 



Shell with the nucleus subtrigonal, inflated ; umbones mesial, elevated, and curved 

 forwards and inwards ; anterior side very wide, rather flattened, giving somewhat a three- 

 sided figure to the nucleus ; surface of the test with fine concentric striae. 



The convexity of the valves is so considerable that the diameter through both is almost 

 equal to that of the height and length, but the length varies with the stages of growth, the 

 younger forms being more produced laterally and less convex ; the posterior side is always 

 rather compressed, and usually exhibits an angle, which passes obliquely from the umbones 

 backwards, but in the more inflated specimens it is obsolete ; the valves appear to fit 

 closely at their circumference, and the ventral border is regularly and elliptically curved. 

 The anterior side is very wide, and somewhat flattened, giving a three-sided aspect to the 

 general figure. The nuclei do not display any concentric stria?, and we have never found 

 the test preserved. 



Locality. It occurs somewhat rarely in the upper beds of the Great Oolite, two miles 

 east of Minchinhampton, but has not been found in the shelly beds of the same 

 formation. 



Lucina, Brug. 1791. 



Gen. Char. Inequilateral, orbicular, posterior side short or truncated, anterior side 

 more produced. Hinge usually with two small cardinal, and two lateral teeth in one valve, 

 one lateral tooth in the other ; ligament external, but deeply excavated. Muscular 

 impressions two in each valve, the anterior one narrow and lengthened, the posterior 

 somewhat rounded ; impression of the mantle not sinuated. 



