BIVALVIA. 169 
Carprra NucuLINA. Duwjard. Mém. de la Soc. Géol. de France, t. ii, part i, p. 265, t. xvili, 
fig. 13 a—f, 1837. 
— epxicua. Id., p..265, t. xvii, fig. 17 a—b. 
MINUTA. Sacchi. Catal. Conch. Reg. Neap., p. 4, figs. 5, 6, 1836. 
VENERICARDIA corBIs. S. Wood. Catalogue, 1840. 
— ANCEPS, var. Id. 
Spec. Char. Testd minutd, ovatd vel trigonuld, obliqud, crassd; apicibus acutis ; stris 
concentricis, densis, undulatis ; sulcis radiantibus, obsoletis. 
Shell small, ovate, or subtriangular, oblique, thick and strong; concentric striz 
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 ;3;ths of an inch. In what may, perhaps, be called the normal 
form (Wuculina, 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. 
