Division J.— LIGAMENT INTERNAL, WITH OR WITHOUT ANY EXTERNAL LIGAMENT. 
MACTRACEA.—7 GeEneERa. 
Shell equivalve, mostly gaping at the lateral extremities. Ligament internal, mith or without any ex- 
ternal ligament. 
I, Tue LIGAMENT ALWAYS INTERNAL. 
(1) Shells gaping at the sides, 
Lurraria.—Shell inequilateral, transversely oblong or rounded, lateral extremities gap- 
ing: hinge with one tooth folded in two; or two teeth, one of which is simple, 
with an adjoining, deltoid, oblique pit, projecting inwards; no lateral teeth. Li- 
gament internal, affixed in the pits. 
(a) Shells transversely oblong. 
Plate IV. Fig. 2. (a) L. elliptica. (Mactra lutraria—Gmel.) 
(b) Hinge of ditto. 
(6) Shells orbicular, or subtriangular. 
Plate IV. Fig. 3, L. compressa. (Mactra Listeri—Gmel.) 
Macrra.—Shell transverse, inequilateral, subtriangular, gaping very little at the sides; 
beaks prominent. One compressed folded cardinal tooth in each valve, with an 
adjoining pit projecting inwards; two compressed entering lateral teeth, near the 
hinge. Ligament internal, inserted in the cardinal pits. 
Plate IV. Fig. 4. (a) M. stultorum. (Idem.—Linn.) 
(b) Hinge of ditto. 
(2) Shells not gaping at the sides. 
CrassATELLA.—Shell inequilateral, suborbicular or transverse, the valves close: two ra- 
ther diverging cardinal teeth, and a pit by the side of them. Ligament internal, 
inserted in the pit of each valve: lateral teeth, none; or obsolete. 
Plate IV. Fig.5. @ C. kingicola. [Mrs. Mawe’s Cabinet.] 
(6) Inside of ditto. 
Erycina.—Shell transverse, rather inequilateral, equivalve, rarely gaping: two unequal, 
diverging cardinal teeth, having a pit between them; and two oblong compressed, 
short, entering lateral teeth. Ligament internal, fixed in the-pits. 
Plate IV. Fig. 6. (a) E. striata. (Sowerby.) 
(6) Inside of ditto. 
II, LIGAMENT SHEWING ITSELF ON THE OUTSIDE, OR BEING DOUBLE; HAS ONE INTERNAL 
THE OTHER EXTERNAL. 
Uncutiwa.—Shell longitudinal or transverse, rounded on the upper part, subequilateral, 
valves not gaping: beaks eroded. A short, and rather bifid cardinal tooth in each 
