3 86 
MOLLUSCS. 
size; Cerithium giganteum, an Eocene form, attaining quite a foot and a half 
in length. 
One of the most curious groups of gastropods is that of the worm-shells 
(Vermetidce ), in which the shells might be mistaken for the tubes of marine 
worms, as they are similarly twisted and attached to stones and other substances 
in the same way. They are free and spiral in early life, and crawl about like 
ordinary gastropods, but they afterwards settle down and become stationary for 
the rest of their existence. In these circumstances, a walking-foot, being of no 
further use, becomes modified into a mere support of the operculum. The animals 
are worm-like, with a short proboscis, horny jaws, and radula, and the head 
supporting two short tentacles, with the eyes at the base. The species are not 
numerous, and occur chiefly in warm and temperate seas. The members of the 
extensive family of the Melaniidce are inhabitants of fresh water, and are 
abundant in all subtropical parts of the globe. The shells are not, as a rule, 
attractive, being clothed with a dark or olivaceous periostracum. Some are long, 
slender, and acute, others quite globular. Perhaps the most remarkable form is 
Tiphobia ltorei, an inhabitant of Lake Tanganyika, in Central Africa. In the 
typical Melania the aperture of the shell is entire, but in some of the other genera, 
such as Melanopsis and Faunus , it is distinctly notched in front. The animal is 
provided with a horny operculum, and many are viviparous. Hundreds of species 
have been described, but many, as is the case in all fresh-water groups, are 
distinguished by very slight differences. The Strepomatidce, or Pleuroceridce, are 
the North American representatives of the Melaniidce of the Eastern Hemisphere, 
from which they are distinguished by the absence of the marginal mantle-fringes, 
and in being oviparous in their mode of reproduction. In certain places they 
abound in such countless numbers as almost to cover the bed of some of the 
streams in Tennessee and Alabama. About five hundred forms have been recognised. 
The genus Io contains the largest and most striking species of all. They are 
short, spindle-shaped shells, often with nodose or spinose whorls, and with the 
aperture prolonged into a distinct anterior canal. They are restricted to certain 
parts of Virginia and Tennessee. The little sea-snails known as periwinkles 
(Littorinidce) are dwellers on the shore. They are all vegetarians, and occur in 
the Arctic and Antarctic regions, as well as in 
temperate and tropical shores, wherever they 
can find rocks and stones to crawl upon. Some 
occur at low water; others live at hiok-water 
mark, or where they are only occasionally 
reached by spring tides. Some ascend the 
mangrove trees, and have been found hundreds 
of yards from the sea. Four species of 
Littorina inhabit the English coast, the 
commonest being the well-known L. littorea, 
which is consumed in such enormous quan¬ 
tities. The periwinkles have horny jaws, and 
a very long radula, sometimes two or three times as long as the animal itself; and 
they are all furnished with a horny operculum to protect themselves with, when 
spawn of periwinkle (magnified). 
