8 
Division I. _ Ligament internal, with or without any external Ligament. 
MACTRACEA 7 Genera. 
Shell equivalve, mostly gaping at the lateral extremities. Ligament internal, with or without any ex- 
ternal ligament. 
I — The Ligament always internal. 
(1) Shells gaping at the sides. 
Lutraria. — 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. 
(«) Shells transversely oblong. 
Plate IV. Fig. 2. (a) L. elliptica. (Mactra lutraria. — Gmel.) 
( b ) Hinge of ditto. 
(b) Shells orbicular, or sub triangular . 
Plate IV. Fig. 3. L. compressa. (Mactra Listeri. — Gmel.) 
Mactra. — 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. (a) C. kingicola. [Mrs. Mawe’s Cabinet.] 
(b) 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. (Sovverby.) 
(£>) Inside of ditto. 
II. — Ligament shewing itself on the outside, or being double; has one internal 
the other external. 
Ungulina. — Shell longitudinal or transverse, rounded on the upper part, subequilateral, 
valves not gaping : beaks eroded. A short, and rather bifid cardinal tooth in each 
