296 NERITID^. 



with a polished horny epidermis. They are mostly confined to 

 the fresh waters of warm regions. One species (N. Jluviatilis) 

 is found in British rivers, and in the brackish water of the Baltic. 

 Another extends its range into the brackish waters of the North 

 American rivers ; and the West Indian N. viridis and meleagrin 

 are found in the sea. 



Some are amphibious, clinging to the roots of Nipah palms 

 and other trees on the margins of rivers, while a few inhabit the 

 foliage of tall trees that overhang the waters. 



NERiTiNA (restricted), Swainson. Shell globular or oval-conic, 

 usually brilliantly ornamented with colors ; inner lip crenulated, 

 rarely simple. Philippines, etc. 



THEODOxus, Montf. (Vitta [Klein], Adams. Puperita, Grray. 

 Elea, Ziegler.) Shell transverse, smooth or nearly so ; inner lip 

 flattened, simple-edged or denticulated. Operculum, peg rudi- 

 mentarj^ Inhabits the fresh waters of Europe. Kobelt has 

 divided this group into neritoglobus, for species of the form of 

 N. jluviatilis (Ixxviii, 66), and neriticonus, for the conical forms, 

 like N. Mertoniana (Ixxviii, 67). 



DOSTIA, Gray. fMitrula, Menke.) Shell sandal-shaped, solid, 

 the apex completely posterior and a little lateral ; peristome con- 

 tinuous and free ; inner lip septiform, arcuated and denticulated 

 in the centre of its margin. Brackish water, East Indies. N. 

 crepidularia. Lam. (Ixxviii, 68). 



CLYPEOLUM, Reel. (Neritella 'Humph.] , Adams.) Shell glob- 

 ular, oval or conic; thin, covered by a corneous epidermis; 

 aperture semilunar; inner lip straight, flattened, smooth or den- 

 ticulated on the margin ; outer lip very full, often produced into 

 a tongue upon the spire posteriorly. Operculum, peg and rib 

 well developed, quite separated from each other. Mostly Polj^- 

 nesian. N. puUigera, Linn. (Ixxviii, 69, YO). 



CLiTHON, Montfort. (Corona, Recluz.) Shell coronated with 

 tubercles, or short or long spines, covered with a corneous epi- 

 dermis ; inner lip usually denticulated, presenting frequently a 

 large superior tooth. Operculum with peg and rib both well 

 developed, connected in half their length. The spines that 

 usually ornament the whorls are tubular, and sometimes very 

 long ; the Clithons inhabit tropical countries ; thej' crawl slowly, 

 and only show during locomotion the tentacles and the tip of 

 the muzzle; they prefer a stony bottom, clear and free from 

 weeds, where the water is tolerably quiet. N. longispina 

 (Ixxviii, 11). 



NERiTONA, Martens. Peg of the operculum depressed, almost 

 flat, lobate at its tip. JSf. labiosa, Sowb. (Ixxviii, 72). Fresh water, 

 Polynesia. Too close to Clypeolum. 



neritodryas. Martens. Rib of the operculum deeply furrowed, 

 multilobate at the tip, and deeply excavated beneath. Living 



