770 CLASS XIII. 
wreathed shell are affixed to it by a muscle which is attached to 
the uppermost turn near the columella, and descends along the turns 
to attach itself to the head by one portion, and to the hinder ex- 
tremity of the foot by the other. This muscle, on its contraction, 
serves to draw the snail into its house. 
Gasteropods advance also by creeping on the ventral disc, or 
foot, in which they fix alternately the fore part, then the hind 
part, to the surface over which they are moving, whilst the disc is 
contracted into sinuous transverse grooves that proceed from be- 
fore backwards. Pteropods swim by means of the wing-like ap- 
pendages that surround the mouth. The same purpose is served by 
the flaps of skin on the body of most Cephalopods, which are 
especially assisted when creeping on the bottom of the sea and in 
ather motions, as seizing their prey, &c. by the muscular arms, 
furnished with suckers, that surround the head. 
Molluses, as to their geographical distribution, are not suf- 
ficiently ascertained, and it is difficult to form any general con- 
clusions from comparison, since all countries, coasts, and seas, have 
not been investigated with equal care. Molluscs are more nume- 
rous than conchifers in great seas and on rocky coasts. In warmer 
regions they surpass the bivalves greatly in multiplicity of species. 
The greatest profusion of marine molluscs, as well as to genera as 
species, belongs, beyond doubt, to the great ocean between the 
west coast of America, and the east coast of Asia and New Hol- 
land. Tropical forms seem to extend more widely in the southern 
than in the northern hemisphere. Amongst the genera that are 
most numerous in species, we may here mention especially Purpura, 
Mitra, Cyprea, and Conus ; the species of the three last genera are 
almost all from the great ocean, and the Hast Indian sea. It is 
remarkable that the molluscs on the western and on the eastern 
coast of South America differ to such a degree, that there is scarcely 
a species that is common to both coasts. The genus Chiton, so 
numerous in species in the great ocean, is represented by a couple 
of species on the east coast of America. Species that dwell at con- 
siderable depths are naturally further dispersed than those which 
live more in the neighbourhood of coasts. Consequently it is not 
sufficient to ascertain the longitude and latitude of the districts 
within which certain species occur; the depth also of the sea ought 
to be ascertained in which they commonly live. With the depth 
