MOLLUSCA. 487 



plete. The penis is in some external, and incapable of being- 

 withdrawn, while in others it is retractile, and situate in a 

 cavity in the right tentaculum. 



The body of the animal is attached to the shell 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 organisation. 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, unfortunately, animals, widely different in structure, 

 inhabit shells of the same form, and vice versa, so that, how- 

 ever 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, 

 therefore, 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 tiie 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, tempoi'ary combiucv 

 tions. 



