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Show 406 MOLLUSCA. LITHO:QOMus, Cuv., In which the shell is oblong, and almost equally rounded at the two ends, the summit being close to ~he anterior extremity. The species of this subgenus at first simply attach themselves to stones like the common Mytili; subsequently, however, they perfot·ate and excavate them in order to form cells, into which they enter, and which they never quit afterwards. Once entered, their byssus ceases to grow( 1). One of them, the Mytilus lithophagus, I...., Chemn., VIII, lxxxii, 729, 730, is very common in the Mediterranean, where from its peppery taste it is esteemed as food. A second, Modiola caudigera, Encyc. pl. 221, f. 8, has a very hard small appendage at the posterior extremity of each valve, which perhaps enables it to excavate its habitation. ANODONTEA, Brug. The anterior angle rounded like the posterio1·, and that next to the anus obtuse and almost rectilinear; the hinge of the thin and mode· rately convex shell has no appearance of a tooth whatever, being merely furnished with a ligament which extends along the whole of its length. The anirnal,-LIMNEA, Poli, has no byssus; its foot, which is very large, compressed and quadrangular, enables it to crawl upon the sand or ooze. The posterior extremity of its mantle is provided with numerous small tentacula. The Anodontes inhabit fresh water. Several species are found in France, one of which-Mytilus cygneus, L., Chcmn., VIII, Ixxxv, 762, is common in ponds, &c., with oozy bottoms. Its light and thin shells a1·e used for (1) M. Sowerby doubts this fact, which is, however, well attested by M. Poli from ocular demonstration-Test. Neap., II, p. 215. The pJ. xxxii ofthc same work, fig. 10, 11, 12, 13, also proves that the animal resembles that ofa Mytilus, and not that of a Plwlas or a Petricola. The mode in which the Litlwdomi, Plwlades, Petricol;e, and some other bivdves perforate stones, has been the subject of much discussion; some of the disputants holding it to be effected by the mechanical action of the valves, and others sim· ply by solution. Sec the Mem. of M. Fleuriau de Dellevue, Journ. de Phys., an X, P· 345; Poli, Test. Neap., U, 215, and Edw. Osler, Phil. Trans. part ill, 1~26, P·. 347. All things considered, the first of these opinions, whatever be the ddnculbes Jt presents, seems to us to come nearest to the truth. ACEPHALA TEST ACEA. 407 milk-skimmers, but its flesh is not eaten on account of its insipidity( I). An oblong species, in which the hinge is granulated throughout its whole length, is distinguished by M. de Lamarck under the name oflRIDINA(2); the hind part of its mantle is somewhat closed(s). Dr Leach distinguishes another by that of DIPSADA, where the angles are more decided, and in which there is a vestige of a tooth on the hinge. UN1o, Brug. These Mollusca resemble the Anodontes both in their animal and shell, with the exception of their hinge, which is more complex. There is a short cavity in the anterior part of the right valve, which receives a short plate or tooth from the left one, and behind it is a long plate which is inserted between two others on the opposite side. They also inhabit fresh water, preferring running streams. Sometimes the anterior tooth is more or less stout and unequal, as in Mya margaritifera, L.; Drap., X, 17, 19. A large thick species, the nacre of which is so beautiful that it is employed as pearls. Found in France; as is the Vnio littoralis, Lam., Drap., X, 20. A smaller and square species. Sometimes the anterior tooth is laminiform, as in the Mya pictorum, L.; Drap., XI, 1, 4. An oblong and thin species known to every one( 4 ). Lamarck distinguishes the HYRIA, Lam., In which the angles are so decided that the shell is nearly triangular( s). (1) Add, M. anatinus, Chemn., VIII, lxxxvi, 763;-M. jluviatilis, List., clvii,. 12;-M. stagnalis, Schrced., Fluv., I, 1;-M. zellensis, lb., II, 1;-M. dubiw, Adans., XVII, 21; and the pl. 201, 202, 203, and 205, of the Encyc. Method., Test. (2) !rid. exotica, Encyc. Method., Test., pl. 204;-Add Irid. nilotica, Caillaud, Voy. a M6ro6, pl. lx, f. 11. {3) See Desl1ayes, Mem. de la Soc. d'Hist. Nat. de Paris, 1827, III, p. 1, pl. 1. (4) Numerous species, remarkable for size or form, inhabit the rivers and lakes of the United States. Messrs Say and Barnes, who have described them, have established some new subgenera among them. · {5) Hyria rugosa, Encyc. Method., pl. 247, 2. ' · |
