ACEPIIALA TESTACEA. 
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Nucula, Lam. — 
Has the teeth of the hinge in a broken line. The form of the shell is elongated and narrowed towards 
the posterior end. We do not know the animal, but it is probably not much unlike that of the pre- 
ceding genus. 
For a long time we have placed here the 2'rigonice, Brug., so remarkable for their hinge, which is 
furnished with two plates en chevron, crenulated on both surfaces, and each penetrating into two 
cavities, or rather between four plates of the opposite side, similarly crenulated on their internal sur- 
faces. From the marks on the inside of the valves we inferred that the animal had not tubes, of any 
length at least; and MM. Quoi and Gaymard having discovered it alive, we find, in fact, that, like 
the Arcacese, it has an open cloak without any separate orifices, not even one for the anus. Its foot 
is large, truncate, and hooked at its anterior part. The recent Trigoniae resemble the Cockles in the 
figure of their shell, and in the manner in which it is ribbed. Their interior is nacred. The fossil 
Trigoniae are considerably ditferent. Their shell is flattened on one side, oblique, longest in the direc- 
tion perpendicular to the hinge, and crossed in the contrary direction by series of tubercles. 
THE SECOND FAMILY OF THE ACEPHALA TESTACEA,— 
The Mytilace^, — 
Has the cloak open in front, but with a separate aperture for the passage of excrements. All of them 
have a foot with which they crawl, or at least draw out, direct, and fix the byssus. They are known 
to the vulgar by the name of Mussels. 
Mussels, properly so called {Mylilus, Linn.), — ■ 
Have a closed, triangular shell, with equal ventricose 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 anterior one, 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 is the anus, opposite which the cloak forms a peculiar aper- 
ture or little tube. The animal {CalUtriche, Poll) has the edge of its cloak provided with branched 
tentacula near the rounded angle, as it is there that the Avater required for respiration enters. In front, 
near the acute angle, there is a small transverse muscle, and a large one behind near the obtuse angle. 
The foot resembles a tongue. 
In Mytilus, Lam., the summits [of the valves] are nearly terminal. Some species are smooth, others striated. 
The common Mussel (M. edulis, Linn.) is spread in extraordinary abundance along- all our coast, where it is often 
suspended, in long clusters, to rocks, piles, ships, &c. It forms an article of food of some importance, but it is 
dangerous when eaten to excess ; [and under certain unknown circumstances, or to some individuals, becomes 
deleterious]. Some species have been found in a fossil state, (which Brongniart distinguishes generically by the 
name Mitiloide). 
In Modiolus, Lam., the apices are lower, and towards the third of the hinge ; they are also more protuberant and 
rounded, whence the shell has more of the ordinary shape of bivalves. We may also distinguish separately the 
Lithodomus, Cuv., which has an oblong shell, almost equally rounded at both ends, and the summits very near 
the anterior. They at first suspend themselves to stones, like the common Mussels, but then they perforate them, 
and bury themselves in the excavations, whence they cannot again issue. After they have made their cells, the 
byssus ceases to grow.-* One species {Mytilus lithophagus, Linn.) is very common in the Mediterranean, where 
it furnishes a food agreeable enough on account of its peppery taste. There is another {Modiola caudigera) which 
has the posterior end of each valve armed with a very hard little appendage, that is, perhaps, of service in the exca- 
vation of its dwelling.f 
The Fresh-water Mussels {Anodontes, Brug.) — 
Have the anterior angle rounded like the posterior ; and the angle near the anus obtuse, and almost 
rectilinear : their thin and moderately ventricose shell has no tooth in the hinge, but merely a liga- 
ment occupying its entire length. The animal {Limncea, Poll) is without a byssus ; and it creeps over 
* "We cannot imagine,” says Sowerby, ” that this remark has 
been made from actual observation, because we believe it to be con- 
trary to the nature of the animal to be at one time attached by a 
byssus, and not at another ; and, moreover, we have ourselves seen 
Lithodomi not more than one-eighth of an inch in length, in as com- 
pletely-formed proportions as the fuller-grown specimens.” — Ed. 
t The means by which the saxicavous bivalved Mollusca perforate 
rocks has given rise to much discussion : some believe that they do 
the work by the meclianical action of the valves ; others attribute it 
to a soivent secreted by the animal. All things considered, I think 
the first of these opinions, notwithstanding the difficulties in the way 
of its adoption, is yet the most probable. 
