GASTROPODS. 



387 



retracted within their shells. The spawn of this species is illustrated in the figure. 

 Nearly allied to the last is the genus Lacuna, in which the animal has long, 

 slender tentacles, while on the upper part of the foot there are two long 

 appendages, which extend much beyond its pointed extremity. The species 

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

 seaweeds. The so-called staircase-shells (Solariidw) are all formed much upon 



horned winkle, Lacuna divaricata (enlarged). 



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 upwards to the 

 apex 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-downwards, 

 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 

 Solariium, and the operculum is very remarkable, being conical, 

 and consisting of many whorls. 



All the molluscs belonging to the Rissoidai are very small. The 

 shells are something like a minute Littorina, but they are often longi- 

 tudinally ribbed, a style of sculpture not met with in the periwinkles. 

 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 

 Hydrobiidai are closely allied to the Rissoidai, and may be regarded as their 

 representatives in fresh and brackish water. Like the periwinkles, although 



Rissoa, 



