_Round Apple Snail._
GENERIC CHARACTER.--See Pl. 103.
SPECIFIC CHARACTER.
_A. testa globosa, laevissima, olivacea; spira depressa; aperturae margine cra.s.so, fulvo, sulcato; umbilico parvo, contracto, juxta basin posito; operculo testaceo._
Sh.e.l.l globose, very smooth, olive; spire depressed; margin of the aperture thick, fulvous, grooved; umbilicus small, contracted, placed near the base; operculum sh.e.l.ly.
Helix Ampullacea. _var._ _Gmelin_, _p._ 3626. _no._ 43. _Chemnitz_, 9 _tab._ 128. _fig._ 1133. 1134. _p._ 105.
This is the most common of the two sh.e.l.ls of this genus, which have their mouths closed by a sh.e.l.ly operculum. It is well described by Chemnitz, and his figures are very tolerable; yet, like all the authors of that period, he considered it as a variety of _Helix ampullacea_. From all these supposed varieties it is, nevertheless, quite distinct; the spire is more depressed than that of any other species, and the umbilicus is placed near the bottom of the inner lip: the whole sh.e.l.l is very smooth, and, although generally of a uniform yellowish olive colour, is sometimes marked by narrow bands of purple brown. The margin of the outer lip is slightly reflected, and the colour, beneath the epidermis, almost white. It is a native of the rivers of India.
From the remarks on this genus, made at Plate 103, the fact of their opercula being either sh.e.l.ly or h.o.r.n.y, is sufficiently established. These formations, however, there is every reason to suppose, may generally be detected by the following indications. In such species as have a sh.e.l.ly operculum, the margin of the aperture is thickened all round, and has a parallel internal groove for its reception: the probable use of this groove I have detailed elsewhere. On the other hand, in those species which are known to have h.o.r.n.y opercula, this margin and groove do not exist; and that part of the sh.e.l.l which is between the top of the aperture and the umbilicus, is thin and unprotected. This latter formation is by far the most frequent, and leads to the conclusion that the majority of these sh.e.l.ls have their opercula h.o.r.n.y.
On the distinctions between this genus and _Planorbis_, little need be said. The princ.i.p.al difference consists in the latter having no operculum; but another, and a very remarkable one, (which seems to have escaped all writers,) is, that the sh.e.l.ls of the latter genus are dest.i.tute of any columella. The _Planorbis cornu-arietis_ of Lamarck, has been removed by Mr. G. Sowerby to this genus. This sh.e.l.l, it is true, appears to be intermediate between one and the other; but the only affinity which it bears to _Ampullaria_, is in the oval form of the aperture; while it is allied to _Planorbis_ by its discoid form, want of the columella, and being universally described as without an operculum: the preponderance of evidence is clearly in favour of the situation originally a.s.signed to it by Lamarck.
The characters, therefore, given to the genus _Ampullaria_ by Mr. G.
Sowerby, will be found incorrect. There was no necessity for explaining, much less for altering, (in this instance,) the masterly definitions of Cuvier and Lamarck. With regard to the second species given by Mr. Sowerby to ill.u.s.trate this genus, he is no less in error; for the real _A. rugosa_, of all authors, is a strikingly distinct sh.e.l.l from that which he has figured under this name. This will be sufficiently obvious by referring to the figures either of Lister, Chemnitz, or Lamarck.
Having offered these remarks on a subject to which I have paid some attention, I wish to refrain from pointedly noticing other errors and misconceptions into which Mr. G. Sowerby has fallen; rather wishing that greater experience, and more matured judgment, may lead him to do this himself, prior to the publication of the system of Conchology which he has announced.