ACEPHALA TESTACEA. 
93 
Trigonia, Brng. 
So remarkable for the hinge, which is furnished with two plates en 
chevron, crenulated on both faces, each of which penetrates into two 
cavities, or rather between four plates of the opposite side, similarly 
crenulated on their internal surface. 
The internal impressions on the shell had already warranted the 
supposition that the animal was not provided with long tubes. Messrs. 
Quoy and Gaymard have lately discovered living specimens of this 
genus, and in fact, its mantle, as in tlie Arcae, is open and without 
any separate orifice, even for the anus. The foot is large, its anterior 
portion trenchant and like a hook. 
The living Trigoniae resemble the Cardiae in the form of their 
shell, and the ribs which furrow it : its interior is composed of 
nacre *. 
The fossil Trigoniae 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 series of tubercles f. 
FAMILY II. 
MYTILACEA. 
In the second family of the testaceous Acephala, the mantle is open 
before, but has a distinct aperture for the faeces. 
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 Muscles. 
Mytilus, Lin. 
The true Mytili or Sea-Muscles have a closed shell, with equal, con- 
vex 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 longest, is the anteriorone, and allows the passage of the byssus ; 
it terminates 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 Callitriche, Poll, has the edges of its 
mantle provided with branched tcntacula 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. 
♦ The Trigonie nacree, Lam., Ann. du Mus., Ixvii, 1. 
•f" Tng. scabra, Encyc. Method,, pi. 237, f- 1 ; — Tr, nodulosn, Ib., 2; — Tr, naris, 
Ib., 3 ; — Tr. aspera, Ib. 4. See also Parkins., Ore:. Rem., Ill, pi. xii. 
