■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 Tovinia 
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 Rissoidce 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 
Hydrobiidcc are closely allied to the Rissoidce, and may be regarded as their 
representatives in fresh and brackish water. Like the periwinkles, although 
