MOLLUSCA+ 179 
by means of two muscles, which adhere to the pillar near 
the same place, and shift their position, by an arrangement 
not well understood, in proportion as the individual increases 
in size. ‘These muscles terminate in the foot and mouth. 
The animals of this order have not been examined suffi- 
ciently in detail, to admit of their distribution into natural 
groups, distinguished by characters founded on important 
differences of organization. The form of the shell has been 
resorted to, with the view of assisting arrangement. The 
characters thus furnished would be useful and valuable, were 
they the index of any peculiar internal structure. But, un- 
fortunately, animals widely different in structure inhabit 
shells of the same form, and vice versa, so that, however 
useful the mere conchologist may find the form of the shell 
to be in his arrangements, it can only be regarded by the 
zoologist as occupying a subordinate place. Without, there- 
fore, entering into any details regarding the structure of the 
few species which have been examined anatomically, we 
shall merely point out the tribes and families which have 
been contemplated, the characters of which in a great mea- 
sure depend on the shape of the shell. 
Ist Tribe. 
Shell external. 
The shelly covering exhibits all the variations of the spi- 
ral form. The internal structure has hitherto been in a 
great measure neglected, so that the characters employed- 
in the methodical distribution of the species and genera are 
derived from the shelly appendage of the cloak. The groups, 
therefore, are merely artificial, temporary combinations, to a 
very few of which only we shall make reference. 
Genus Janrumna.—Foot with an adhering spongy body. 
In this genus, represented by the Helix janthina of Lin- 
