58 MOLLUSCA. 



NA, Lam. 



This genus has lately been separated from the CyclostomFP, because 

 there is no ridge round the aperture of the shell ; because there is a 

 small angle to that aperture as well as to the operculum. arid finally, 

 because the animal, being provided with branchia-, inhabits the 

 water, like all other genera of this family. It has a very short snout 

 and two pointed tcntacula ; eyes at the external base of the latter, but 

 on no particular pedicle, arid a small membranous wing on each side 

 of the fore part of the body. The anterior edge of the foot is double, 

 and the wing of the right side forms a little canal which introduces 

 water into the respiratory cavity, the incipient indication of the siphon 

 in the following family. 



The common species, Helix rivipara, L. ; Drap., I, 16, whose 

 smooth and greenish shell is marked with two or three purple, 

 longitudinal bands, and which abounds in stagnant waters, in 

 France, produces living young ones : in the spring of the year 

 they may be found in the oviduct of the female, in every stage 

 of development. Spallanzani assures us that if the young ones 

 be taken at the moment of birth and be reared separately, they 

 will reproduce without fecundation, like those of the Aphis. 

 The males, however, are nearly as common as the females ; they 

 have a large penis which protrudes and retracts, as in Helix, 

 but through a hole pierced in the right tentaculum, a circum- 

 stance which renders that tentaculum apparently larger than 

 the other, and which furnishes us with a mode of recognizing 

 the male*. 



The Ocean produces some shells whicli only differ from the Palu- 

 dinse in being thick. They form the 



LITTORIXA, Feruss., 



Of which the common species, Le Vigneau Turbo littareus, L., 

 Chemn. V, clxxxv, 1852, abounds on the coast of France, where it is 

 eaten. The shell is round, brown, and longitudinally streaked with 

 blackish. The 



MONODON, Lam. 



Only differs from Littorina in having a blunt ami slightly salient 

 tooth at the base of the colu.mella, which sometimes has a (so a fine 

 notch. The external edge of the aperture is crenulated in several 

 species. The animal is more highly ornamented, and is generally 

 furnished with three or four filaments, on each side, as long as its 

 tentacula, The eyes are planted on particular pedicles at the exter- 

 nal base of the tcntacula; the operculum is round and horny. 



* Add, Cyclost. achatinum, Drnp. I, 18; ('. itn^untm, Id., 19, 20, or 

 tentaculata, L., &c. ; and the small species of salt-water ponds described by Beu- 

 dant, Ann. du Mus., XV, p. 199. 



