GASTEEOPODA. 261 



SheU tliick, smooth or spirally grooved; epidermis horny; 

 outer lijD thickened and sometimes denticulated 

 within ; columella broad and flat, with its inner f^f-J^ 

 edge straight and toothed; operculum shelly, 

 Fig. 112. 



Bisfrihution, 173 species. Nearly all warm seas. 

 West Indies, Eed Sea, Zanzibar, Philippines, Fig. ii2.» 

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

 American species dwell in the streams; one species at the 

 Philippines sometimes climbs up trees. 



i^oss/?, 60 species. Lias — . Britain, &c. The palaeozoic nerites 

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

 2')iIeo]jsis. 



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

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

 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. 



Distrihution, 1 species. Pacific. 



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



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

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

 tially enveloping the spire. 



PiLEOLTJS (Cookson), J. Sowerby. 



Etymology, inleolus, a little cap. 



Type, P. plicatus (PL 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. 



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

 Ancliffe, and Minchinhamj^ton, 3 species, P. neritoides is a 

 neritina, 



Neritina, Lam. Fresh-water nerite. 



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

 (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 iV. jpeloronta. West Indies. 



