14 
SHELL GALLERY. 
25-26. 
27-28. 
Cases 
31-32. 
placed at the base of the tentacles instead of at their tips, they 
have a long proboscis armed with a different rasping tongue (radula), 
a spiral operculum, and the sexes are distinct, whereas the true 
Snails are hermaphrodite. 
The operculated air-breathers have been divided into many 
sections, chiefly on account of differences in the apertures of the 
shells and in the opercula. They most abound in hot countries, but 
a few species are met with in temperate regions. 
The " Apple-Snails " (Ampullariidce) live in the rivers and 
marshes of tropical regions, and, although represented by a large 
number of species, exhibit comparatively slight variations in form 
and colour. The animal has both a pectinated gill and a lung 
cavity, being thus enabled to breathe either water or air. 
Fig. 8. 
Cases 
32-35. 
Ampullaria canaliculata. 
The " Periwinkles " (Littorinidw) are found almost on every 
known shore ; they feed upon all kinds of marine vegetation. 
Some species are met with at low-water mark, others on rocks 
almost beyond the reach of the sea, and some have been discovered 
inland nearly half a mile away from the shore. It is calculated that 
1900 tons of the " Common Periwinkle " {Littorina littorea), of the 
value of £15,000, are annually consumed in London alone. 
The family of Calyptrwidce includes the " Slipper-Limpets " 
(Crepidula) and the " Cup-and-saucer Limpets" (Crucibulum). 
Although furnished with a foot, they rarely crawl about, but 
remain attached to rocks, stones, or other shells, sometimes 
forming a shelly plate under the foot by which they become 
fixed to the spot where they have taken up their abode. 
The " Cowry-shells " {Cyprmdce) are remarkable for their varied 
