104 CLASS ACEPHALA. 
The fossil trigonia are different. Their shell is flattened 
on one side, oblique, longest in a direction perpendicular to 
the hinge, and traversed in a contrary direction by a series 
of tubercles. (Trig. Scabra, Encyc. Meth.) 
In the second family of the testaceous acephala, the 
MYTILACEA, 
The mantle is open before, but has a distinct aperture for the 
feeces. ; 
All these bivalves have a foot, used in crawling, or at least, 
serving to draw out, direct, and place the byssus. They are 
commonly known under the generic name of mussels. 
MytTIuus, Lin. 
The true mytili, or ‘sea-mussels, have a closed shell, with 
equal, convex, and triangular valves. One of the sides of the 
acute angle forms the hinge, and is furnished with a long 
narrow ligament. The head of the animal is in the acute 
angle; the other side of the shell, which is the largest, is the 
anterior one, and allows the passage of the byssus ; it termi- 
nates in a rounded angle, and the third side ascends towards 
the hinge, to which it is joined by an obtuse angle. Near 
this latter is the anus, opposite to which the mantle forms an 
opening or small particular tube. The animal (CALLI- 
TRICHE, Poli,) has the edges of its mantle provided with 
branched tentacula, near the rounded angle, as it is there that 
the water enters required for respiration. Before, and near 
the acute angle, is a small transverse muscle, and a large one 
behind, near the obtuse angle. Its foot resembles a tongue. 
In the true mytili the summit is close to the acute angle. 
Some of them are striated and others smooth. 
Myt. Edulis, L. This common mussel is frequently seen 
suspended in extended clusters, along the whole coast of 
France, to rocks, piles, &c. It forms a considerable item of 
