GASTEllOrOPA. 261 



Shell thick, smooth or spirally ,2:rooved ; epidermis horny ; 

 outer lip thickened and sometimes denticulated 

 within ; columella broad and flat, with its inner 

 edge straight and toothed; operculum shelly, 

 Fig. 112. 



Distrihidion, 173 species. Nearly all warm seas. 

 West Indies, Eed Sea, Zanzibar, Philippines, Fig. 112.* 

 Australia, Pacific, West America. (Cuming.) Many of the 

 American species dwell in the streams; one species at the 

 Philippines sometimes climbs up trees. 



Fossil, 60 species. Lias — . Britain, &c. The palaeozoic nerites 

 are referred by D'Orbigny to turho, natica, &c. N. haliotis is a 

 pileo2:>sis. 



Sub-genera. Neritoma, Morris, 1849. N. sinuosa, Sby. 

 Portland stone, Swindon. (Mus., Lowe.) Shell ventricose, 

 thick ; apex eroded ; aperture with a notch in the middle of the 

 outer lip. Casts of this shell are common, and exhibit the 

 condition of the interior characteristic of all the nerites ; it was 

 probably fresh water. 



Neritopsis, Grateloup. N. radula (PI. YIIL, Fig. 9). Shell 

 like nerita ; inner lip with a single notch in the centre. 



Distrihidion, 1 species. Pacific. 



Fossil, 20 species. Trias ? Britain, France, &c. 



Velates, Montf. N. perversa, Gm. (PL IX., Fig. 36). Inner 

 lip very thick and callous ; outer lip prolonged behind, and par- 

 tially enveloping the spire. 



PiLEOLTJS (Cookson), J. Sowerby. 



Etymology, pileolus, a little cap. 



Type, P. pHcatus (PI. IX., Figs. 37, 38). 



Shell limpet-like above, with a sub-central apex ; concave 

 beneath, with a small semi-lunar aperture, and a columellar 

 disc, surrounded by a broad, continuous peristome. 



Distribution, marine ; only known as fossils of the Bath oolite, 

 Anclifi'e, and Miuchinhampton, 3 species, P. neritoides is a 

 neritina. 



Neritina, Lam. Fresh-water nerite. 



Examples, N. zebra (PI. IX., Fig. 39), N. crepidularia 

 (PI. IX., Fig. 40). 



Shell rather thick at the aperture, but extensively absorbed 

 inside ; outer lip acute ; inner straight, denticulated ; operculum 



* Fig. 112. Operculum of N. peloronta. West Indies. 



