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Show 404 }.10LLUSCA. NuouLA, Lam. The Nucul.-e are Arc::e, in which the teeth are arranged on a broken line. Their form is elongated, and narrowed near the pos. teriot· extremity. Their animal is unknown, but is probably not far removed from those of the preceding shells( 1 ). This has long been the place assigned to the TRIGONIA, Brug. 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 &urface. The internal impressions on the shell had already warranted the supposition that the animal was not provided with long tubes. Messrs Quay and Gaymard have lately discovered living specimens of this genus, and in fact, its mantle, as in the Arc::e, is open and without any separate orifice, even for the anus. The foot is large, its anterior portion trenchant and like a hook. The. 1i ving Trigonic:e resemble the Cardic:e in the form of their shell, and the ribs which furrow it: its interior is composed of nacre(2). The fossil Trigonic:e 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 tubet·cles(3). FAMILY II. MYTILACEA. In the second family of the testaceous Acephala, the mantle is open before, but has a distinct aperture for the freces. 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. (1) .llrca pellucida, Chemn., VII, liv, 541;-.ll.rca rostrata, L., Jd., lv, 550, 551;. ll.rc. pella, lb., 546;-.ll.rc. nucleus, Id., lviii, 574. (2) The Trigonie nacree, Lam., Ann. du Mus. IV, lxvii, 1. (3) Trig. scabra, Encyc. Method., pl. 237, f. 1; Tr. nodulosa, Jb., 2;-Tr. navis, lb., 3;-Tr. aspera, lb. 4· See also Parkins., Org. Rem., III, pl. xii. ACEPHALA TESTACEA. 405 M YTILus, Lin. The true Mytili or Sea-Muscles have a closed shell, with equal, convex 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 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 latter is the anus, opposite to which the mantle forms an opening or small particular tube. The animal-C.ALLITRIOHE, Poli, has the edges of its mantle provided with branched tentacula near the rounded angle, as it is there that . the water enter~ 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 re-sembles a tongue. In the true Mytili the summit is close to the acute angle. Some of them are striated and others smooth. Myt. edulis, L. This common Muscle is frequently seen suspended iu extended clusters, along the whole coa.st of France, to rocks, piles, &c. &c. It forms a considerable item of food, but is dangerous if eaten to excess(l ). Some of them are found fossil(2). In the MoDIOLUS, Lam. Separated from the Mytili by Lamarck, the summit is lower and near the third of the hinge. This summit is also more salient and rounded, approximating the Modioli more closely to the ordinary form of the bivalves(3). We may also separate from the Mytili the (1) Add Mytilus barbatus, L., Chemn., VIII, lxxxiv, 749;-.M. angulatus, Ib., 756;-M. bidens, Ib., 742, 745;-M. afer, lb., lxxxiii, 739-741;-M. smaragdinus, lb., 745;-M. versicolor, Jb., 748;-.M. lineatus, 753;-.M. exustus, lb., 754;-M. &triatulus, lb., 744;-M. bilocularis, lb., lxxxii, 736;-.M. vulgaris, lb., 732;-.M. 8e:tatilis, Rumph., Mus. xlvi, D;-.M. Julgidus, Argenv., xxii, D; probably the same as the Mya p e1·na, Gm., Chemn., VIII, lxxxiii, 738;-M. azureus, lb. H;M. murinus, Jb., K;-.J.'I:f. puniceus, Adans., I, xv, 2;-M. niger, lb., 3;-M· lawigatus, lb ., 4, &c.: some of these, however, may be mere varieties. (2) M. Brongniart has formed them into a. subgenus by the name of MYTILOID.l, Ap. Cuv. Oss. Foss. tome II, pl. iii, f. 4 . (3) Mytilu~ modiolus, Chemn., VIII, b:xxv, 757-760, and that of Mull., Zool. Lan., II, lii, which appears to be another species;-.M. discors, Chemn., VIII, lxxxiv, 764-768;-M. testaceus, Knorr., Vergn., IV, v, 4, &c. |