Lamarck’s Genera of Shells. 317 
4, Hiatella*. 
Shell equivalve, very inequilateral, transverse, gaping at the 
superior margin. Hinge with one small tooth on the right valve, 
and two, rather larger, oblique teeth on the left. Ligament 
external. 
One species. Hiatellaarcticat. (Myaarctica. Linn.) 
Shell transversely oblong; anterior side the longest; apex 
truncated ; the two angles of the valves muricated ; one very 
oblique ; striz transverse. North Seas. Pl. vi. Fig. 62. 
5. Isocardia ¢. 
Shell equivalve, cordate, ventricose ; beaks distant, diverging, 
involute-spiral. Two flattened, entering, cardinal teeth, one 
eurved and sunk under the beak; one elongated lateral tooth, 
situated below the corselet. Ligament external, forked on one 
side. 
Type. Isocardia Cor §. (Chama Cor. Linn.) 
‘Shell cordate-globular, smooth, yellow; nates whitish. 
European Ocean, Mediterranean, &c. 4 Species. PI. vi. 
Fig. 63. 
7th Family. 
Arcacga. (4 genera). 
Cardinal teeth small, numerous, entering, and disposed on 
each valve in a straight, arched, or broken line. 
The arcacea are very remarkable by the hinge of their shells, 
which are equivalve, regular, the beaks commonly distant, the 
ligament entirely external, and the muscular impressions lateral. 
Some are transverse, others rounded. Several have a more or 
less velvety epidermis. Some adhere to rocks by tendinous 
threads, which the animal fixes to them, and the shell gapes, 
more or less, at the superior margin. 
The Arcacea generally bury themselves in the sand, at a short 
distance from the shore, and are all sea shells. 
* Dim. from hiatus—gaping—the little gaper? + Arctic. 
+ From io, equal, and xaedia, a heart. § Heart. 
