Order 1. 



ACEPIIALA TESTACEA. 



367 



NucuLA, Lam, — 

 Has the teetli 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 Trhjonice, Brug., so remarkable for their hinge, which is 

 furnished with two plates en chevron, cienulated 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 Arcaccjc, 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 Trigonise resemble the Cockles in the 

 figure of their shell, and in the manner in which it is ribbed. Their interior is nacred. The fossil 

 Trigoniaj arc considerably different. Tlieir 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 ACEPIIALA TESTACEA,— 



The MYTILACEiE, — 



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 {Callitriche, Poll) has the edge of its cloak provided with branched 

 tentacula near the rounded angle, as it is there that the water 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. ediilh; Linn.) is spread in extraordinary abundance along all our coast, where it is often 

 suspended, in lon^ 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, beconics 

 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 

 I'oundcd, \Yhence the shell has more of the ordinary shape of bivalves. AVe may also distinguish separately the 

 Lithodomns, 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 lithophagns, Linn.) is very common in the Mediterranean, where 

 it furnishes a food agreeable enough on account of its peppery taste. There is another (,ModioIa caudii/crn) whicli 

 has the posterior end of each valve armed with a very hard little appendage, that is, perhaps, of service in the e.\ca 

 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 : tlieir thin and moderately ventricose shell has no tooth in the hinge, but merely a liga- 

 ment occupying its entire length. The animal {Limncsa, Poll) is without a byssus ; and it creeps over 



* "We cannot imagine," says Sowerby, " that tliis 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-eiijhth of an inch in length, in as com- 

 pletcly-formed proportions as the fuller-grown specin\ens,"— Ed. 



t The means by which the saxicavous bivalved MoUusca perforate 

 rochs has given rise to much discussion : s»uie believe that they do 

 the wuxM by the mechanical action of the valves ; others attribute it 

 to a solvent secreted by the animal. All things considered, I tliiiik 

 the first of these opinions, notwithstanding the diflicultirs in the way 

 of its adoption, is yet the most probable. 



