MOLLUSC A. 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. 



1st 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 JANTHINA Foot with an adhering spongy body. 

 In this genus, represented by the Helix janthina of Lin- 



