BIVALVIA. 169 



Cakdita nuculina. Dujard. Mem. de la Soc. Geol. de France, t. ii, part ii, p. 265, t. xviii, 

 fig. 13 a—f, 1837. 

 — exigua. Id., p. 265, t. xviii, fig. 17 a — b. 

 MiNUTA. Sacchi. Catal. Conch. Reg. Neap., p. 4, figs. 5, 6, 1836. 

 Venericaedia corbis. S. Wood. Cp,talogue, 1840. 

 — ANCEPS, var. Id. 



Spec. Char. Testa minutd, ovatd vel trigonuld, obliqud, crassd; apicibus acutis ; striis 

 concentricis, densis, undulatis ; sulcis radiantihus, obsoletis. 



Shell small, ovate, or subtriangular, oblique, thick and strong ; concentric striae 

 or ridges thick and undulating ; radiating sulci obsolete. 



Diameter, \ of an inch. 



Localities. Cor. Crag, Sutton. 



Red Crag, Walton-on-the Naze. 



This is one of the commonest shells in the Coralline Crag at Sutton, and the two 

 valves are often found united. Among the numerous specimens that are in my 

 cabinet, the two forms figured by Dujardin from the Touraine beds may be dis- 

 tinguished. In that variety which has the greatest diameter from the umbo to the 

 ventral margin, (which appears to be the one now living in the Mediterranean, as given 

 by Philippi,) the shell is most ventricose, the umbo most prominent, and the concentric 

 ridges much more elevated and distinct, than those are which have more distinct 

 radiations from the umbo ; in the other extreme form of variation which I had called 

 anceps {exigua, Dujard.), the shell is more compressed, with a greater proportional 

 diameter from the anterior to the posterior side, and the radiating ridges are more visible 

 to the naked eye, and this variety appears to have attained rather larger dimensions, 

 measuring as much as -j^ths of an inch. In what may, perhaps, be called the normal 

 form [Nuculina, Dujard.), the exterior is covered with concentric ridges at rather 

 unequal distances, and in the young state, or at the umbo, they are very wide apart; they 

 appear to undulate, or are made uneven by the rays which cross them, but the rays 

 fade away so imperceptibly into those in which they are obsolete, that I am unable to 

 draw a line between the two. Our shell possesses one large tooth in the right valve, 

 of a somewhat triangular form, stretching out towards the posterior, and in the other 

 valve there are two teeth, one smaller, immediately beneath the umbo, of a triangular 

 shape, the other compressed and elongated, sloping posteriorly, and nearly parallel to 

 the dorsal margin ; the muscle marks are large in comparison with the size of the 

 shell. 



