BIVALVIA. 81 
Since the original establishment of the genus by Lamarck, it has been much 
curtailed, and is now restricted to those shells witha pectiniform or denticulated hinge, 
having the posterior portion, as it were, cut off; the lines of denticulations forming 
nearly a right angle, and the animal being without the posterior siphonal tubes ; 
consequently there is no indentation in the impression formed by the muscles of the 
mantle. 
The genus thus restricted is in a recent state rather sparingly distributed, although 
found in the seas of both hemispheres. As fossil, it has been obtained low in the 
Secondary Formations. 
The species in a living condition are inhabitants of the sea at all depths, some being 
found near low water mark, while others are truly pelagian, and have been observed in 
the deepest regions Mollusca are known to frequent. Mr. Garner, in his ‘ History 
of the Lamellibranchiata,’ says, there is a distinct pinnate process in the mantle of 
the animal, for the purpose of secreting the numerous teeth of the hinge; these teeth 
are prominent, sharp pointed, and angular, the angle being directed towards the 
umbo from both sides. 
1. Nucuta tavieata, J. Sowerby. Tab. X, fig. 8, a—é. 
NucuLa LaHvieata. J. Sow. Min. Conch., t. 192, figs. 1, 2, 1818. 
os — Goldf. Pet. Germ., vol. 1i, p. 157, pl. 125, fig. 19, a—e. 
= — S. Wood. Must. Mag. Nat. Hist., New Series, vol. iv, p. 296, 1840. 
— — Id. Catalogue, 1840. 
— — Nyst. Add. & la Faune Conch. de Belg. (Bull. Acad. de Brux, t. ix, 
p. 450), 1842. 
— — Morris. Cat. Brit. Foss., p. 94, 1843. 
— _— Nyst. Coq. Foss. de Belg., p. 228, pl. 17, fig. 8, a—b, 1844. 
Spec. Char. Testa transversd, ovatd, valde inequilaterd, levigatd, tenui, mar- 
garitaced, claus; anticé brevi, subangulatd ; postice productiore, rotundatd ; margine 
ventrali integerrimo. 
Shell, transverse, ovate, very inequilateral, smooth, thin, nacreous, and closed ; 
anterior side short, slopmg, or angulated; posterior much produced and rounded ; 
ventral margin without crenulations. 
Longest diameter, 12ths of an inch; eight, 1 inch. 
Locality. Cor. Crag, Sutton. 
Red Crag, Walton Naze. 
This is the largest species of the genus that I am yaccinintcl with, either in the 
recent or fossil state, it appears to have attained its full development in the Red 
Crag, as in the Deposits of that Period at Walton Naze specimens are by no means 
rare ; it is found also in the older or Coralline Crag, where, however, they are few in 
number and small in size. 
M. Deshayes has quoted this as synonymous with JV. ovata, an Eocene species from 
11 
