BIVALVES___MACTRA. 33 
MACTRA. — KNEADING-TROUGH. 
DESCRIPTION OF PLATE VIII. 
Drv. l—Fam. 1. Fig. 1. M. carinata. 
Fam. 1. Fig. 3. M. cinerea. 
Fam. 1. Fig. 5. M. stultorum. 
Fam. 2. Fig. 2. M. violacea. 
Fam. 2. Fig. 4. M. subtruncata. 
Div. II.—Fam. 2. Fig. 6. M. listeri. 
Fam. 3. Fig. 7. M. lutraria. 
Shell bivalve, nequilateral, equivalve: middle tooth of the hinge 
complicated, with a small hollow on each side; lateral ones re- 
mote, and inserted into each other. 
THE genus Mactra possesses little beauty or variety. 
The number of species amounts to forty-two, and a 
similarity of coloring and form pervades the whole. 
The shape of the Mactra is subtriangular, or oblong. 
The exterior is generally smooth, or delicately striated; 
but in some species it is transversely wrinkled or rib- 
bed. The valves ina few specimens gape at both ends, 
in others at the anterior only, and some have both * 
ends closed. These characters form the distinctions of 
the divisims and families of the genus. 
The most prevailing color is lilac, or white tinged with 
blue or yellow, and some species have delicate rays of 
purple upon a brown ground. The Mactre havea 
semipellucid appearance, and are generally thin, brittle, 
and remarkably light. 
The hinge of the Mactra, by which it is distinguished 
from all the other genera of Bivalves, is of a triangular 
D 
