RISELLA. 317 



The eyes in Modulus are situated half way up the ten- 

 tacles, the a}3erture of the shell is not pearly within, and 

 the inner lip is furnished with a strong tooth ; the opercu- 

 lum, moreover, is nearly circular, and is composed of 

 several vthorls. 



Species of Modulus. 



angulatus, C. B. Adams. cluplicatus, A. Adams. 



candidus, Petit. lenticularis, Chem. 



catenulatus, Phil. obliquatus, A. Adams. 



cerodes, A. Adams. tectum, Gmel. 



disculus, Pliil. unidens, Lister. 



Genus RISELLA, Gray. 



Eyes situated on the tentacles. 



Operculum ovate, subspiral. 



Shell trochiforra, with aflat or concave base, axis imper- 

 forate, whorls flattened, the last angulated, often acutely 

 keeled ; aperture depressed, oblique, rhombic, dark or 

 variegated internally; outer lip acute, simple. 



Syn. Bembicium, Phil. 



Ex. R. lutea, Quoij and Gaimard, pi. 33, fig. 5. Oper- 

 culum, R. melanostoma, Gmelin, fig. 5, «, 5, h. Shell, R. 

 melanostoma, fig. 5, c. 



This genus is known among the Littorinidee by its im- 

 perforate, top-shaped shell ; the absence of eye-peduncles 

 and lateral membrane of the foot, and the shell not being 

 pearly within, prevents it from being confounded with the 

 Trochidce.. 



