In the selection of generic characters, sufficiently important to separate Ampullaria from Paludina, great difficulty at present exists; as the fundamental principle on which they should be founded (the formation of the animal) is entirely wanting. It is only known that these shells, like the Paludinæ, are furnished with an operculum. The absence or presence of this organ has been found of the first generic importance; though the substance of which it is composed, as well as the form it assumes, can be considered only as indicating specific distinctions. This is proved from the fact, that among the Naticæ some have horny, and some shelly, opercula: in Phasianella, this part is, in some species, almost flat, in others remarkably convex; in Turbo, Lam. its form is even more variable, and in the present genus a similar uncertainty exists. One species alone has been positively described as having this part shelly, while in two others the operculum is as certainly known to be horny; to these last may be added a third, found by myself in the lakes of Pernambuco in Brazil, but to which I have not immediate access. The shells here figured were, however, received from the same place by Mrs. Mawe, and, I think, are of the identical species. The spire is sometimes worn, and the whole shell very thin.
AMPULLARIA fasciata,
Banded Apple Snail.
Generic Character.
- Testa globosa, umbilicata. Spira depressa vel brevissima. Apertura integra, magna, ovata. Operculum testaceum vel corneum. Animal fluviatile.
- Shell globose, umbilicated. Spire depressed or very short. Aperture entire, large, oval. Operculum shelly or horny. Animal fluviatile.—Generic Type Helix ampullacea Lin.
Specific Character.
- A. testâ ovato-globosâ, olivaceâ, fasciis obscuro-purpureis angustis ornatâ; spirâ brevi, levatâ, apice acuto; labii margine tenui; umbilico mediocri.
- Shell ovate-globose, olive, with narrow bands of obscure purple; spire short, elevated, the tip acute; margin of the lip thin; umbilicus moderate.
- Am. fasciata. En. Meth. pl. 457. f. 3. f. 4. (reversed and young).
- Helix ampullacea. Linn. Lister, 130. f. 30. Seba, t. 38. f. 1 to 6, 58, 59. Chemnitz, 9. t. 128. f. 1135. Gualt. t. 1. R.
Several fossil shells of this genus are mentioned as existing in the extinct volcanoes of Ronca, in bituminous marl near Pont St. Esprit, &c. as quoted (on the authority of the illustrious Cuvier) by Mr. Bowdich.