GASTKOPODA. 17 
The Melaniidce are freshwater Snails which abound in most tropi- Cases 
cal and subtropical countries ; about 1000 species are known. They 38 ~ 4L 
are mostly of dark colours, and are fond of muddy places. 
The Cerithiidce are chiefly marine forms, some, however, entering Cases 
brackish water. About five hundred fossil species have been described, 
some of them gigantic in comparison with any now living, of which 
more than two hundred are known. 
The Scala scalaris was formerly considered a great rarity, as Case 44. 
much as £40 having been given for a single speci- 
men, which might now be purchased for as many lg ' * 
pence. 
The " Worm-shells " ( Vermetidce) are a very ti£k Cases 
peculiar family. Their shells can scarcely be dis- 
tinguished from the shelly tubes which are formed 
by certain species of marine worms, Serpula, &c. 
They are free and spiral in early life, but after- 
wards become distorted and generally attached to 
rocks, stones, &c. A foot for walking purposes 
therefore would be of no use ; consequently it 
is more or less obsolete, serving only as a support Scala scalaris. 
to the operculum. Ca8e 41 - 
The " Screw-shells " (TurriteTUdce,) have elongate tapering shells ; Case 48. 
about 100 recent and 200 fossil species are known. One species only 
(Turritella communis) is now found living on the British coasts. 
The Xenophoridcz have the singular habit of cementing to the case 49. 
exterior of their shell, stones, pieces of coral, and fragments of other 
shells ; hence they have been called " Carrier-shells," and, according 
to the kind of material chosen, have been named " Conchologists " 
and " Mineralogists." Beyond acting as a disguise, and consequently 
as a protection, there does not appear to be any special utility in 
thus adding to the weight of their own shells. The animals do not 
glide like most other molluscs, but scramble along like the Strombs, 
the form of their foot being small, divided into a front, expanded, 
and a hind, tapering portion admirably adapted to the nature of the 
ground on which they live, which usually consists of broken and dead 
shells. 
The " Wing-shells " (Strombidce,) are the largest of the Gastro- Cases 
pods with a proboscis or non-retractile snout. They do not crawl 49_52 - 
like most other Gastropods, but progress by a sort of hopping move- 
ment. They act as scavengers, feeding on decomposing animal matter. 
The Strombus gigas, or " Fountain-shell," occurs in great numbers 
c 
