BIVALVIA. 81 



Since the original establishment of the genus by Lamarck, it has been much 

 curtailed, and is now restricted to those shells with a 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. Nucula laevigata, /. Soioerby. Tab. X, fig. 8, a — b. 



Nucula LiEViGATA. /. Sow. Min. Concli., t. 192, figs. 1, 2, 1818. 



— — Gold/. Pet. Germ., vol. ii, p. 157, pi. 125, fig. 19, a — c. 



— — 5. Wood. Illust. Mag. Nat. Hist., New Series, vol. iv, p. 296, 1840. 



— — Id. Catalogue, 1840. 



— — Nyst. Add. a 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, pi. 17, fig. 8, a— b, 1844. 



Spec. Char. Testa transversa, ovatd, valde inaquilaterd, lavigatd, tenui, mar- 

 garitaced, clansd ; antice brevi, subangulatd ; postice productiore, rotundatd ; margine 

 ventrali integerrimo. 



Shell, transverse, ovate, very inequilateral, smooth, thin, nacreous, and closed ; 

 anterior side short, sloping, or angulated ; posterior much produced and rounded ; 

 ventral margin without crenulations. 



Longest diameter, lfths of an inch; height, 1 inch. 



Locality. Cor. Crag, Sutton. . 



Red Crag, Walton Naze. 



This is the largest species of the genus that I am acquainted 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 N. ovata, an Eocene species from 



11 



