Volume Ii Part 12 (2/2)
The form of the notch or sinus which terminates this part will also be found of much importance in discriminating the species; for no sh.e.l.ls vary more in their form, thickness, or convexity than these do, according to their locality, age, or other circ.u.mstances.
Sh.e.l.l transverse, oval; rather thick and ventricose; both extremities obtuse; the anterior side (from the umbones to the exterior margin) obliquely rounded; umbones prominent; hinge margin rather thick, slightly curved, and swelled immediately under the umbones; sinus short, abrupt, curved; epidermis coa.r.s.e, black, and much wrinkled; inside stained with yellow, and having a narrow reddish rim or margin.
For this species, now, I believe, first made known, I am indebted to G. C.
Bainbridge, Esq. of Liverpool, who received several specimens from the United States. It appears to have been unknown to Mr. Say, who has published an account of the land and river sh.e.l.ls of North America.
The student might be led to suppose, that the two genera of _Unio_ and _Anodon_ are strongly characterized; for the first includes many of the most ponderous bivalves yet discovered, and the second some remarkably thin and brittle. Among the _Uniones_ are sh.e.l.ls furnished with hinges of the greatest force, while most of the _Anodons_ are perfectly dest.i.tute of any; nevertheless, the gradations by which these characters approach each other are very remarkable, and some sh.e.l.ls which partake of both have been arranged in separate genera. Of these, the best defined are _Hyria_ Lam.
and _Dipsas_ of Leach; the one allied nearest to _Unio_, but having the cardinal teeth a.s.suming the appearance of lateral or lamellar teeth; the other more resembling the Anodons, but furnished with a strongly defined and elevated lamellar tooth, extending the whole length of the hinge.
Between these two genera should be placed another of Lamarck's, called by him _Iridinia_, which has likewise only a simple lamellar plate, but broken into a great number of crenated teeth. The observing Mr. Say has likewise proposed another under the name of _Alasmodonta_, which, however, I shall take another opportunity of noticing.
I have ventured to exchange the ungrammatical name of _Anodonta_, given by Bruguiere to this genus, for _Anodon_, at the suggestion of the learned Dr.
Goodall, Provost of Eton College.
Pl. 97
[Ill.u.s.tration]
MARGINELLA,
_Date Sh.e.l.l._
GENERIC CHARACTER.
_Testa ovata. Spira brevissima aut nulla. Labium exterius cra.s.sissimum, margine interiore crenato. Columella plicata. Basis subintegra. Animal capitatum, capitis fronte profunde emarginato; oculis ad tentaculorum 2 subulatorum basin externam adsitis; tuba jugulari simplici; pede magno, foliaceo, pone attenuato; penula dilatata, testae latera obtegente._
Typ. Gen. _Volutae perspicula, glabella, prunum, &c._ Lin.
Sh.e.l.l oval. Spire very short or concealed. Exterior lip very thick, with the internal margin crenated. Pillar plaited. Base nearly entire.
Animal capitate; head notched in front, with lengthened, pointed tentacula, at the external base of which are the eyes; neck with a simple tube; foot large, foliaceous, pointed behind; mantle dilated, and folded over the sides of the sh.e.l.l.
Generic Types _Vol. perspicula, glabella, prunum, &c._ Lin.
Lamarck first separated the sh.e.l.ls comprised in this genus from the Volutes of Linnaeus; their princ.i.p.al distinction rests in the formation of the outer lip, which has a very thick margin, more or less toothed on the inner rim; the base likewise is nearly entire, and the inner lip quite wanting.
By these peculiarities, the _Date Sh.e.l.ls_ are easily known from the _Volutes_ on the one hand, and the _Cowries_ on the other; and the invaluable researches of M. Adanson, who has described and figured the animals of each of these genera, has established this distinction on the most solid principles; it will, however, be interesting to trace, by the sh.e.l.ls only, how beautifully this arrangement is developed.
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