FAMILY NERITACEA. 201 



part fluviatile, of essentially tropical origin. One small species pre- 

 vails throughout the temperate fresh waters of the Eastern Hemi- 

 sphere ; there is no strictly marine species north of Senegal. In 

 the Western Hemisphere there is no species of either habit north of 

 California. The animal is small, nearly covered by a comparatively 

 massive shell, of rarely more than three whorls, brilliantly painted, 

 and often enveloped by a rather fibrous epidermis. The head is 

 produced into a short proboscis, and the eyes are peculiar in being 

 raised on short slender stalks detached from the tentacles, while 

 the branchiae appear in the form of parallel rounded plates, set on a 

 long triangular membrane, which is partially free. The animal 

 encloses itself firmly in the shell, by means of an operculum which 

 hinges at one corner on the columella by a kind of apophysis. 

 The Neritacea are about two hundred and fifty in number, comprised 

 in three genera, Nerita, Neritina, and Navicella, inhabiting chiefly 

 the seashore and rivers of the Polynesian and Malayan Archipe- 

 lagos. Of the river Neritacea, which are the more numerous, we 

 have one species in Britain. 

 It belongs to the genus : — 



1. Neritina. Animal carrying an obliquely ovate shell, of three 

 whorls, having a tranverse semilunar aperture. 



<r 



Neritina fluviatilis. {Moderately Enlarged.) 



Genus I. NERITINA. Lamarch. 



Animal ; short, carrying a solid few-whorled shell, head produced 

 into a proboscis, which is comparatively short, tentacles con- 

 tractile, slender, flesuous, margined with dark lines, having the 

 eyes at their base on short detached stalks, foot oblong, obtuse 

 before and behind. Branchiae free, for the greater part con- 

 sisting of a long triangular acute membrane, on the sides of 



