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Show 416 MOLLUSCA. ever, are wanting, and the anterior muscular impression is not so long(l ). The genus VENUs, Lin. Comprises many Testacea whose general character consists in the teeth and plates of the hinge being approximated under the summit, in a single group. They are usually more flattened and elongated, in a direction parallel to the hinge, than the Cardia. The ribs, when there are any, are almost always parallel to the edges, being dil·ectly the reverse of their arrangement in the Cardia. The ligament frequently leaves an elliptical impression behind the summits, which has received the appellation of vulva, and before these same summits there is almost always an oval impression term-ed the anus or lunula(2). The animal is always furnished with two more or less protractile tubes, sometimes united, and with a compressed foot, which enable it to crawl. M. Lamarck appropriates the name of VENUS to those which have three small diverging teeth under the summit. This character is particularly well marked in the oblong and slightly convex spe· cies(3). Some of them-the AsTARTJE, Sowerb., or CRASSINJE, Lam.,-have only two diverging teeth on the hinge, and approach the Crassatellce in their thickness and some other characters( 4). Among the cordiform species, that is, those which arc shorter and have more convex nates, and with more closely approximated teeth, we should remark those where the plates or transverse stri;e terminate in crests(5) or tuberosities(6), and those that have longi· tudinal ribs and crests elevated behind. (1) Ungulina transversa, Kam., Sowerb., Gen. of Shells, No. X. (2) These fantastic appellations of vulva and anus, have probably caused the extremity of the shell, which corresponds to the true anus of the animal, to be styled the anterior, and that where the mouth is situated, the posterior. We have restored to these extremities their true denominations. We must recollect that the ligament is always on the posterior side ofthe summits. (3} Venus litterata, Chemn., VII, xli;-V. rotunda, lb., xlii, 441;-V: textiliJ, lb., 442;-V: decussata, xliii, 456, &c. (4) Venusscotica, Hans Lerin, VIII, tab. 2, f. 3;-Crassina danmoniensis,Lam.; and among the fossil species, .flst. lucida, Sower., Min. Conch., IT, pl. 137, f. 1;. flst. Osmalii, Lajonkere, Soc. d'Hist. Nat. de Paris, I, t!lb. 6, f. 1. (5) Venus dyaera, Chemn., VI, 27, 299;-Ven. plicata, Encyc· pl. 275, 3, a, b; -Ven. crebimlica, lb., f. 4, 5, 6. (6) Venus puerpera, Encyc., 278;- Ven. corbis, Lam., Encyc. pl. 276, f. 4. ACEPHALA TEST ACEA. 417 We subsequently and gradually come to the CYTHERE.IE, Lam., which have a fourth tooth on the right valve, projecting under the lanula, and received into a corresponding cavity in the right one. Some of them h.ave an elliptical and elongated form(I); others are convex(2), and it IS among these latter that we must place a celebrated s~e.cit:s (Venus Dio~e, ~·· Chemn., VI, 27, 271), from whose form Origmated the application of the name Venus to the genus. Its transvel'Se plates terminate behind in salient and pointed spines. There are s~me.~peci~s of an. orbicular form, and with slightly hooked summits, m which the Impression of the retractor of the tubes forms a large and almost rectilinear triangle(3). When their animals are better known, we shall most probably have to separate from the Cytherere, I. Those species of a compressed lenticular form, in which the nates are united into a single point. The fold of the contour of the mantle is wanting·, and shows that their tubes are not protractile( 4); 2. Those of a convexly orbicular form, in which the fold is not onlywanting, but where, as in the Lucinre, the impression of the anterior muscle is very long( s); 3. The thick species with radiated ribs, in which the fold is also wanting, and which connect the genus Venus with that of the Venericardia(6). In the CAPSA, Brug. . Already separated from the former, there are two teeth on the ~mge o~ one side, and a single, but bifid one on the other; the lunula 18 wantmg, the shell convex and the fold, indicative of the retractor of the foot, considerable(7). PETRICOLA, Lam. Also separated from the same genus; the Petricolre, on each side ~ave two or three very distinct teeth on the hinge, one of which is' ork.ed •• The shell is more or less cordiform, but as they inhabit the lllter10r of s t ones, 1· t someti· mes b ecomes very I•r regular. J udg- (l.J P'ents8 gigantea, Encyc., 28, 3;-Ven. chione, Chemn., VI, 32, 343;- Ven. trycma, lb., 347;-Ven. maculata Ib ~3 .~45 (2) Vi. • , ., v 'v • ( 3 ) ~n. meretr~.x,--Ven. lusoria,--Yen. castremis. ( 4 ) Vi.enus ex_oleta, Chemn., VIT, 38, 404-the genus 0RBICULus, Megerle. ( en. Bcrtpta, Chemn., VII, 40,422 . 5) P'en t' . ( 6 ) Vi. • tgrt~ Chemn., VII, 37, :190;-Ven.punctata, lb. 397. (i) Vi en. pectmata, Chemn., VII, 39, 419-the genus ARTRJ:Mrs, Oken. en. tlefo»-ata, Chemn., IV, ix, 79-82. VoL. II.-3 C |
