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.