112 DIMYARIA. MACTRA. 
side of the hinge, this space is divided by a rib into two 
portions, the outer half of the ligament is externally visible 
when the valves are closed. 
Crassatella sulcata. Plate IX. fig. 16. 
The Crassatelle are marine shells, and principally inhabit the coasts of New 
Holland. 
Fossil species are met with in the London Clay, and Calcaire-grossier, near 
Paris. 
Genus XLVIII. — THETIS. — Sowerby. 
Generic Character. — Shell equivalve, subequilateral, 
more or less orbicular and convex; ligament marginal ; 
hinge with three or four accuminated cardinal teeth, but 
destitute of lateral teeth ; pallial impression with a deep 
sinus extending neatly to the beak ; muscular impressions 
round, small and remote from the hinge; ligament external. 
Thetis minor. Plate VII. figs. 22, and 25. 
The shells of this genus are all fossil, and occur principally in the lower 
greensand of Sussex, Lyme Regis, and Isle of Wight. 
SUBDIVISION III. — Ligament internal ; shell gaping at the sides. 
Genus XLIX. — MACTRA. — Lnnneus. 
Generic Character.— Shell generally thin, sometimes 
thick ; equivalve, for the most part nearly equilateral, and 
more or less regularly triangular ; slightly gaping at one 
end, and almost imperceptibly so at the other; each valve 
with one V shaped cardinal tooth, the point being next the 
umbo, and diverging from it, and in some species the limbs 
are disunited at the base, so as to give the appearance of 
two distinct teeth; close on the posterior side is situate a 
very thin sharp tooth ; immediately behind the angular 
tooth is situate the pit for the reception of the ligament, 
and projecting somewhat within the shell ; one valve with 
two lateral teeth on each side, and one on both sides in the 
other, diverging from the beaks, placed near the margin of 
the shell and fitting into the space between the two in the 
opposite valve; two lateral, remote muscular impressions ; 
mantle muscular impression with a small sinus ; ligament 
consisting of two portions, the one considerably larger than 
the other and internal, and the other half external. 
Mactra semisulcata. Plate X. fig. 11. 
In the thick species the lateral teeth are perpendicularly striated ; these are 
generally elongated, with the inner ones more prominent than the outer; 
they are quite short in some species, such as the M, Spengleri. The beaks are 
