FRONT-GILLED GROUP 



3357 



illustrated occurs on the British coasts, and, like the periwinkles, feeds upon sea- 

 weeds. The so-called staircase shells (Solariid<z) are all formed much upon one 

 and the same plan. They are more or less conical, flattened, and umbilicated 

 beneath, and nearly always beautifully sculptured. In Solarium the umbilicus is 

 often very wide, and is bordered by a crenulated edge, winding upward to the apex 



_e 



HORNED WINKLE, Lacuna divaricata. 

 (Enlarged.) 



of the shell, and terminating in a notch upon the aperture. The embryonic shell 

 is sinistral, and in the course of growth becomes turned spire downward, and, with 

 the exception of its base, concealed by the succeeding whorls. About twenty 

 species are known, and are chiefly met with in tropical seas. In Torinia the shells 

 are more narrowly umbilicated, and not carinate at the margin like Solarium, and 

 the operculum is very remarkable, being conical, and consisting of 

 many whorls. 



All the mollusks belonging to the Rissoidce are very small. 

 The shells are something like a minute Littorina, but they are often 

 longitudinally ribbed, a style of sculpture not met with in the peri- 

 winkles. They are found everywhere and live upon algae. A large 

 number of species have been described, and doubtless many forms from 

 all parts of the world remain to be discovered. Even upon the 

 English shores between twenty and thirty forms are met with. This 

 family has been divided into a number of genera and subgenera, partly 

 on differences in the soft parts, and partly upon conchological grounds. The 

 HydrobiidcB are closely allied to the Rissoidce, and may be regarded as their repre- 

 sentatives in fresh and brackish water. Like the periwinkles, although furnished 

 with gills, they are more or less amphibious, passing part of their time out of the 

 water. Four are British. 



Rissoa. 



