352 Div. 2. MOLLUSCA— GASTEROPOUES. Class 3. 



The Ampullaria, Lam. — 



Has a roundish ventricose shell with a sliort spire, like most of the Helices ; its aperture is higher thaa 



wide, furnished with a [calcareous] operculum, and the columella umhili- 

 cated. They live in the fresh and hrackish water of hot climates. The 

 animal has long tentacula, and pedunculated eyes. At the bottom of the 

 respiratory sac, by the side of the long branchial comb, there is, according 

 to the observations of MM. Quoy and Gaymard, a large pouch filled with 

 air, and which may possibly be a swimming bladder. 



The Lanistes, Montf., are Ampullariae with a wide spiral umbilicus. — The Heli- 

 cina, Lam., from the shell, would seem to be .^mpullarioe with the rim of the aper- 

 ture reflected. When this rim is sharp, the shells are Ampullines, Blainv., and 

 when it is blunt, the Olygine of Say. There is one species (Ilelicina neritella. 

 Lam.) remarkable for a white shelly edge on the inner side of the operculum. It 

 appears that the organs of respiration are similar to Cyclostoma, and that the 

 \g. , .— mp jria rugosa animals can live in the open air. [ The Helicinae are land shells. Mr. Gray has 



given a monograph of the genus in the 1st vol. of the Zoological Journal; but since its publication, the number of 



species has been doubled.] 



The Melanin — 

 Have a thicker shell, with the aperture deeper than wide, which expands at the part opposite the spire. 

 The columella has neither fold nor umbilicus. The spire varies greatly in its length. They live in 

 rivers, but there is no species in France. The animal has long tentacula, and the eyes are placed about 

 a third way up on their outer side. 



The Rissoa, Freminv. (Acmea, Hartm.) differs from Melania in having the rim of the aperture united all round. 

 [" All we have met with are littoral shells, and several species abound on our shores." — Sutcerby. — Melanopsis, 

 Ferussac, with nearly the same form as Melania, has a callosity at the columella, and a vestige of an emargination 

 near the base of the aperture, indicating a relationship with Terebra. The Pirena, Lam., have not merely this 

 sinus, but another on the opposite side. Like the Melania, the two last subgenera live in the rivers of the south 

 of Europe, and of warm countries, [" and yet most of the fossil species are found in beds that are considered by 

 geologists, in this country, to be of marine formation." — Sowcrby.'] 



We incline to refer to this place in the system two genera separated from the Volutes, and which 

 have a considerable similarity to Auricula, but are operculated, and have only two tentacula. First, 

 Acteon, Montf., {Tornatella, Lam.), with a convolute shell ; and, secondly, Pyramidella, Lam., with a 

 turreted shell, whose columella is obliquely twisted and plaited. 



The Janthina*, Lam. — 

 Is widely separated from all that precede by the form of the animal. The shell has some resemblance 

 to our land snails, but the aperture is angular at its lower part and at its outer side, where, however, 

 the angle formed by the union of the upper and lower halves of the outer lip, is much rounded in most 

 of the species, and somewhat so iu the common one : the columella straight and elongated, the inner 

 lili turned back over it. The animal has no operculum, but carries under its foot a vesicular organ, 

 like a congeries of foam-bubbles, of solid consistency, that prevents creeping, but serves as a buoy to 

 support it at the surface of the water. The head is a cylindrical proboscis ; and is terminated with a 

 mouth cleft vertically, and armed with little curved spines : on each side of it is a forked tentaculum. 

 The shells are of a violet colour ; and when the animal is irritated it pours forth an excretion of deeper 

 blue to tinge the sea aro\ind it. 



The Litiopa, Hang, is a small conoid shell without an operculum, the body-whorl larger than the spire, and the 

 aperture entire. The animal lives on the gulf-weed, whence it can suspend itself by a thread like a spider from 

 a ceiling ; and by the same thread it can remount at pleasure to the surface of the weed.] 



The Nerita, Linn. — 

 Are shells with the columella in a straight line, which renders their aperture semicircular or semiclliptical. 

 It is generally large in proijortion to the shell, but always closed perfectly with an operculum. The 

 spire is almost obsolete, and the shell scmi-globular.t 



Naficie, Lam., are Neritae with an umbilicus. The animal of such as are known has a large foot, simple tentacula, 

 the eyes sessile at their bases, and a horny [or shelly] operculum. [In Neritopsis, Sowerby, there is abroad notch 

 or smus in the columella, which distinguishes it from Natica and Nerita, whose forms it seems to combine in itself.] 



• M. de Dl.iiiivillc makes tliis genus his family Oiystoma. t Ttc gciiui Nerita, Linn., constitute liic family Htmicycloitoma of BlaiiiviUe. 



