Round Apple Snail.
Generic Character.—See Pl. 103.
- A. testâ globosâ, lævissimâ, olivaceâ; spirâ depressâ; aperturæ margine crasso, fulvo, sulcato; umbilico parvo, contracto, juxta basin posito; operculo testaceo.
- Shell globose, very smooth, olive; spire depressed; margin of the aperture thick, fulvous, grooved; umbilicus small, contracted, placed near the base; operculum shelly.
- 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 shells of this genus, which have their mouths closed by a shelly 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 shell 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 shelly or horny, 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 shelly 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 horny opercula, this margin and groove do not exist; and that part of the shell 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 shells have their opercula horny.
On the distinctions between this genus and Planorbis, little need be said. The principal 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 shells of the latter genus are destitute of any columella. The Planorbis cornu-arietis of Lamarck, has been removed by Mr. G. Sowerby to this genus. This shell, 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 assigned 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 illustrate this genus, he is no less in error; for the real A. rugosa, of all authors, is a strikingly distinct shell 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.