BIVALVIA. 241 
Generic Character. Shell transverse, subequilateral, equivalve ; ovate, elliptical, or 
subtriangular, sometimes thin, occasionally thick and ponderous, slightly gaping 
laterally ; surface smooth, or finely striated concentrically. Hinge with a double 
cardinal tooth in the left valve, and a triangular space for its reception in the right, 
with large and elongated lateral teeth on each side: a large triangular pit for the 
cartilage. Ligament small, external. Impression by the mantle sinuated. 
Animal with its mantle freely open almost as far backward as the siphons ; margins 
fringed ; siphons large and extensile, united to their extremities, and the orifices 
ornamented with cirri; foot long and strong, tongue-shaped, geniculated. 
This genus, constituting a well marked group, has not been much dismembered 
since the time of its original establishment by Linnzeus: a division under the name of 
Spisula, has, however, been proposed by Dr. Gray for those species which have the 
ligament entirely hidden by the dorsal margin, and the more complete union of the 
cartilage with the ligament, and he has reserved the name of J/actra for the species 
which have a small shelly plate interposed between the two portions of the ligament, 
and where it is partially visible when the valves are closed; this distinction is so 
very ill defined that it is not thought necessary the separation should be here adopted. 
The most distinguishing character is the possession of a sharp diverging or chevron- 
formed tooth, somewhat resembling the letter A reversed, and this is placed imme- 
diately beneath the umbo before the cartilage. ‘Some of the species have the lateral 
teeth and the sides of the sockets, into which they are inserted, covered with rugosities 
or ridges in a direction at right angles to these teeth; others have these parts 
quite smooth ; the thicker species are generally furnished with those ridges, but the 
thinner ones are not always free or naked: these might almost constitute a sectional 
distinction, but the same differences are observable in the genera Astarte and Cyrena. 
The species of this genus have been found in most parts of the world, living 
principally in shallow water; they are generally buried in the sand, within a short 
distance from the surface. 
In the fossil state they have been obtained in the Lias and Greensand formations, 
and one is recorded as from the Carboniferous series; but this latter is of doubtful 
character. The species, as well as individuals, are largely developed in the newer 
Tertiaries, and are abundant also at the present day. 
l. Mactra eiauca, Born. Tab. XXIII, fig. 2, a, 4. 
Mactra auauca. Born, Test. Mus. Ces. Vind., p. 51, t. 3, figs. 11, 12, 1780. 
—  HELVA SEU HELVACEA. Chemn. Conch., Cab., vol. vi, p. 234, t. 23, figs. 232, 233. 
= HELVACEA. Payr. Cat. Moll. Cors., p. 29, 1826. 
— _ Phil. En. Moll. Sic., vol. i, p. 10; vol. ii, p. 9. 
— — Forb. and Hanl. Hist. Brit. Moll., vol. i, p. 366, pl. 23, fig. 2, 1848. 
— Neapouitana. Poli. ‘Test. Sicil., vol. i, t. 18, figs. 1—3, 1791. 
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