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 

 other motions, as seizing their prey, &c. by the muscular arms, 

 furnished with suckers, that surround the head. 



Molluscs, 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, Cyprcea, and Conus / the species of the three last genera are 

 almost all from the great ocean, and the East 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 



