ACEPHALA TESTACEA. 
379 
The Venusid^ {Venus, 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, 
which 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 anus.^ 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. de 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 (Asiarte, Sow., or Crassina, 
I Lam.) have only two diverging hinge teeth, and resemble the Crassatella in their thickness and some other 
i 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 j 
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 
J spines that guard its posterior end. There are species too of an orbicular form with slightly curved beaks, in 
which the impression of the retractor muscle of the tubes forms a large, almost rectilinear triangle, 
j When the animals are better known, it is probable we may have to separate from Cytherea,—!. The species of a 
I much compressed, lenticular shape, with beaks approximating to a point. Thei-e being no impression of the fold of 
I the cloak, we infer that the tubes are not extensile. 2. Those of a ventricose, orbicular form, which want the 
I impression just mentioned, but have a very long imprint of the anterior muscle, as in Lucina. 3. The thick species 
“i with radiated ribs and without the impression of the cloak, which connect the Venusidfe with the Venericardia. 
! There has been already separated from Venus the Capsa, Brug., which have on one side of the hinge two teeth, 
j and on the other one only, but bifid ; the shell has no anus, is considerably convex, oblong, and the impression 
I 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 Corhulce, 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 S 2 )henia, Turton, separated from Corbula, and which has C. rostrata as its type, has not 
I been adopted by foreign Conchologists. Sowerby unites it to Mya.] 
I The Mactraid^ {Mactra, Linn.) — 
) Are distinguished among the shells of this family because the ligament is internal, and is lodged on 
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.t In the Lavignons \Listera, 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 j 
posterior side of the shell is the shortest. The valves gape a little. The tubes are separate and very long, as in 
Tellina. One species (Mya Mspanica, 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 j 
opposite end is prolonged into a double tube, that can be pushed far beyond the shell. This is always 
species. The same author has also given a good definition of Amphi- 
desma, which is not synonymous witli tlie LiguUi ; but our limits 
prevent us going into detail. Cumingia, 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 lateral teeth. 
The species are found in sand, in the fissures of rocks, and, so far as is 
known, they are tropical.] 
* These terms are apt to mislead, and are otherwise objectionable, 
j The student should remember that the ligament is always on the pos- 
terior side of the beaks. 
t Erycina, Lam., is allied to Mactra, but indiflferently character- 
ized. One portion of them may be Crassatella:. Amphidesma, Lam., 
II or Lignla of Montagu, appear also to be affined to Mactra ; but they 
I are too little known to assign to them a definite place. \Erycina has 
'I been since well defined by Sowerby, who has characterized three 
