Order 1. 



ACEPHALA TESTACEA. 



371 



The Vendsid^ {Ve7ius, Linn.) — 

 Comprise many shells, whose common character is to have the teeth and laminae of the hinge collected 

 under the beaks in a single group. They are in general flatter and more elongated in a direction 

 parallel with the hinge than the Cardia. Their ribs, when there are any, are almost always transverse, 

 wliich is the contrary of the rule in the Cardia. The ligament often leaves, behind the beaks, an 

 elliptical impression, to which the term vulva has been applied ; and in front of the beaks there is 

 almost always another oval impression that has been called the atius.*' The animal has always two tubes, 

 capable of being more or less protruded beyond the shell, but they are sometimes united together 

 apparently in one ; and it has also a compressed foot wherewith to crawl. 



M. cie Lamarck restricts the name Venus to those which have three divergent teeth under the beaks. This cha- 

 racter is peculiarly distinct in the species with an oblong, slightly convex shell. [These have been separated by 

 Sowerby to form his genus Pullastra, to which he unites the Venerupis, Lam., believing that the latter do never 

 perforate rocks, but merely occupy the holes excavated by other animals.] Some (Aslarte, Sow., or Crassina, 

 Lam.) have only two diverging hinge teeth, and resemble the Crassatella in their thickness and some other 

 characters. Among the heart-shaped species it is important to notice those whose transverse ribs or striae termi- 

 nate in crests or tuberosities on the posterior side ; and those which have longitudinal ribs and elevated crests. 

 They lead by degrees to the Cytherea, Lam., which has a fourth tooth upon the right valve, projecting under the 

 anus, and received in a corresponding fossa of the left valve. There are some species, as in Venus, of an elliptical 

 and elongated form, and others that are ventricose, among which is the famous species (Venus Dione, Linn.), that 

 originated the application of the name of the Goddess of Love to a shell, and remarkable for the long pointed 

 spines that guard its posterior end. There are species too of an orbicular form with slightly cun'ed beaks, in 

 which the impression of the retractor muscle of the tubes forms a large, almost rectilinear triangle. 



When the animals are better known, it is probable we may have to separate from Cytherea,—!. The species of a 

 much compressed, lenticular shape, with beaks approximating to a point. There being no impression of the fold of 

 the cloak, we infer that the tubes are not extensile. 2. Those of a ventricose, orbicular form, which want the 

 impression just mentioned, but have a very long imprint of the anterior muscle, as in Lucina. 3. The thick species 

 with radiated ribs and without the impression of the cloak, which connect the Venusidce with the Venericardia. 



There has been already separated from Venus the Capsa, Brug., which have on one side of the hinge two teeth, 

 and on the other one only, but bifid ; the shell has no anus, is considerably convex, oblong, and the impression 

 left by the retractor muscle of the foot is considerable ; and the Petricola, Lam., with two or three very distinct 

 teeth, one of them forked, on each side of the hinge. Their form is more or less cordate ; but, as they live in 

 cavities of stone, [which they themselves perforate,] they become sometimes irregular. From the marks left on 

 the shell by the cloak, their tubes ought to be larger. 



The Corbulce, Brug., similar in form to the triangular or heart-shaped Cythereae, have only a single strong tooth 

 in each valve, locking side by side. The ligament is internal. The tubes ought to be short; and the valves are 

 rarely quite equal. The fossil species are much more numerous than those actually existing. Some live in the 

 interior of stones. [The Sphenia, Turton, separated from Corbula, and which has C rosirata as its type, has not 

 been adopted by foreign Conchologists. Sowerby unites it to Mya.] 



The MACTRAiDiE {Mactra, Linn.) — 

 Are distinguished among the shells of this family because the ligament is internal, and is lodged on 

 both sides in a triangular fossa. They have all a compressed foot, fit to creep with. 



In Mactra, Lam., the ligament is attended in the left valve, on both sides, with a lateral tooth, which locks 

 within two laminae of the opposite valve. Close to the ligament there is on both valves a tooth which is folded 

 into the shape of the letter V, the point being nearest the umbo. The tubes are short and united. We have some 

 species on our shores.f In the Lavignons [Lisiera, Turton] the lateral teeth are almost obliterated : nothing is 

 noticeable but a small tooth near the internal ligament, and we may remark also a small exterior ligament : the 

 posterior side of the shell is the shortest. The valves gape a little. The tubes are separate and very long, as in 

 TeUina. One species (Mya hispanica, Chemn.) is native, living in the sand at the depth of several inches. 



THE FIFTH FAMILY OF THE ACEPHALA TESTACEA— 

 The Inclusa, — 

 Has the cloak open at the anterior end, or near the middle only, for the passage of the foot. The 

 opposite end is prolonged into a double tube, that can be pushed far beyond the shell. This is always 



* These terms are apt to mislead, and are otherwise objcctioualile. 

 The student should remember that the ligament is always on the pos- 

 terior side of the beaks. 



t Eryriiifij Lam., is allied to Mactra, but indifferently character- 

 ized. One portion of them may be Crassatellse. Afnphidesma^ Lam., 

 or Li^w/« of Montagu, appear also to be affined to Mactra ; but they 

 are too little known to assign to them a definite place. [Erycina has 

 been since well defined by Sowerby, who has characterized three 



species. The same author has also given a good definition of Amphi- 

 desma, which is not synonymous with the Liguln i but our limits 

 prevent us going into detail. Cmningia, Sowerby, should be placed 

 near to Amphidesma. It is remarkable for the dissimilarity of the 

 hinge of the two valves, one having a strong lateral tooth on each side 

 of the ligament, and the other being entirely destitute of iateial teeth. 

 The species are found inaand, in the fissures of rocks, and, so far as is 

 known, they are tropical ] 



