COMMONEB FORMS OF SHELLS. 13 



the result ? We increase the length, decrease the 

 width, and cause the whorls to hecome more diagonal 

 than horizontal. Deepen the sutures, and we ohtain 

 in effect a rough model of another genus, Paludina, 

 of which we have two species P. contecta, with a 

 glossy shell of a yellowish or greenish-hrown colour, 

 with bands and stria3 of a darker brown ; and 

 P. vivipam, which is somewhat smaller, less glossy, 

 the whorls less swollen, the sutures less deep, and 

 the mouth less circular. Both species inhabit ponds 

 and rivers. 



If we now elongate and attenuate the spire, lessen 

 the depth of the sutures, and imagine the shell to 

 have become so much thinner as to be almost trans- 

 parent, we get a notion of Limnaea, a widely-distri- 

 buted, marsh-loving genus, of which there are some 

 eight species in Great Britain. They are all found 

 in still and shallow waters, the best known amongst 

 them being L. stagnalis, auricularia, peregra and 

 palustris. The reproductive system of the Limnceidce 

 and other families of the order Pulmonobranchiata 

 is very singular. The individuals of species which 

 possess an operculum are of different sexes, while in 

 those which have no operculum the sexes are united in 



