128 CLASS III. GASTEROPODA. ORDER VII. PECTINIBRANCHIATA. 
estimation of opercula: the operculum of a mollusk, however, forms no 
essential part of its organization, and we feel convinced that it cannot be 
relied upon as a generic character. The genus Paludestrina of D’Orbigny 
is intended for the reception of those varieties which have their eyes situ- 
ated at the base of the tentacles instead of at the summit ; their operculum 
also differs in being spiral instead of being concentric (vide Pl. CXCVII. 
Fig. 5.). 
The shell of Paludina is described as being ovate or conoidal, and 
covered with an olivaceous epidermis ; the spire is somewhat turrited, 
often eroded ; the whorls are round, either smooth or carinated, and the 
aperture is also round or ovate, the margin being continuous, and the lip 
simple and acute. The operculum is horny and orbicular, either spiral 
or waved. 
The Paludine are found in great abundance in the great rivers of India 
and America, particularly near the sea, and in places where the water is 
a little brackish. 
Examples. 
Pl. CXCGVIL. Fig: 1 and 1*. 
PaLUDINA ACHATINA, Lamarck, Anim. sans vert., new edit., vol. vill. 
p- 512. Encyclopédie Méthodique, pl. 458. f. 1. a, b. 
Helix fasciata, Gmelin. 
Nerita fasciata, Miller. 
Cyclostoma achatinum, Draparnaud. 
Pi. CXCVII. Fig. 2. 
PaLupINA PonpEROsA, Say, American Conchology, pl. 30. f.1. Des- 
hayes, new edition of Lamarck, vol. vii. p. 516. 
Pl. CXCVII. Fig. 3. 
PaLUDINA OLIVACEA, Sowerby, Genera of Shells, No. 41. 
Pl. CXCVIL. Fig. 4. 
PALUDINA UNICARINATA, Sowerby, Genera of Shells, No. 41. 
