ACEPHALA TESTACEA. 



375 



NucuLA, Lam. — 

 lias 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 Tnyonia, Brug., so remarkable for their hinge, which is 

 furnished with two plates en chevron, crenuiated on both surfaces, and each penetrating into two 

 cavities, or rather between four plates of the opposite side, similarly crenuiated 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 Arcace^e, 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 naered. The fossil 

 Trigoniae are considerably different. Their shell is flattened on one side, obhque, 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 MYTiLACEJi, — 



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 {Myiilus, 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 wliieb 

 it is joined by an obtuse angle ; near this is the anus, opposite which the cloak forms a peculi;ix aper- 

 ture or little tube. The animal [CaUUric/ie, Poli) 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 M)jt'dus, Lam., the summits [of the valves] are nearly terminal. Some species are smooth, others striated. 

 The common Mussel {M. eilalis, 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 

 dang"eioas 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 MUiloide). 



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 

 Lilhodomus, Cuv., which has an oblong shell, almost equally rounded at both ends, and the sumiiiits 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 {Mi/iilus Vdhophagus, Linn.) is very common in the Mediterranean, where 

 it furnishes a food agreeable enough on account of its peppery taste. There is another (Modiola coudiyera) 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 dweUing.t 



The Fresh-water Mussels {Anodontes, Brug.) — 

 Have the anterior angle rounded like the posterior ; and the angle near the anus obtuse, and almost 

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

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



* "We cannot imagine," says Sowerby, 

 been niiide from actunl observation, because \ 

 trnjj- to the nature of the unimal to be at o' 

 byssus, and not at another ; and, moreover, ' 

 Lithodomi not more than one-eighth if an inch in length, in as c 

 pletcl3'-formfd prnportiuns iis the fuller-grown specimens."— Ed. 



believe it to be cou- 

 time attaehed by a 

 have ourselves seen 



t The means by which the sa.\icavous bivalved Mollusi 

 rocks has fjiven rise to mueh diseussion : some believe ll 

 the work by the meehaniuiil action of the valves ; others 

 to a solvent secreted by the animal. All things coiiside 

 the first of these opinions, notwithstanding the difficultjfs 

 of its adoption, is yet the most probable. 



perforate 

 Lt they d.. 



