112 MOLLUSCA. 



neritella, (Lister, Conch, tab. 61. fig. 59), is the representa- 

 tive of the genus. 



In the genus MELANIA of Lamark, the shells are turret- 

 ed, longer than broad, effuse at the base, with a twisted 

 solid pillar. The H. amarula is the type. 



The genus MELANOPSIS was instituted by Daudebard to 

 include the shells termed melanias by Olivier in his voyage 

 to the Levant. The mouth is lanceolate, the pillar trun- 

 cated and emarginated above with a callosity at the base. 



29. NERITA. This genus has been subdivided by Ad- 

 anson and Bruguiere into NERITA and NATICA. In the 

 former there is no umbilicus as in the N; exuvia, and, in the 

 latter, there is an umbilicus, as in the N.^anrena. Of the 

 restricted genus nerita, we possess two species; the littoralis, 

 common on our shores, and N. virginea. There are several spe- 

 cies of natica of British growth, the largest of which is the 

 glaucina. The fresh water species have been formed by 

 Lamark, with great propriety, into a distinct genus, under 

 the title Neritina. The N. jftuviatilis occurs in the English 

 rivers. 



30. HALYOTIS. This genus has been dismembered of 

 those species which are destitute of the perforations on the 

 disc. These have been formed into a new genus termed 

 STOMATIA. 



31. PATELLA. This genus, which at first sight appears 

 so very natural, contains shells which exhibit considerable 

 differences, both in form and structure, when narrowly ex- 

 amined. Geoffroy, with great propriety, separated the flu- 

 viatile species under the generic title ANCYLUS, a genus af- 

 terwards employed by Miiller. The animal is essentially 

 distinct from the marine patellae. There are two species 



