64 MOLLUSCA. 



has been more or less attended to by succeeding concholo- 

 gists. It is certainly a very general character, and at first 

 sight might be supposed worthy of forming some of the 

 higher divisions. It appears but rarely in the land shells, 

 more frequently in fresh water shells, and generally in the 

 marine species. It does not, as yet, appear to be connect- 

 ed with any peculiar organization, although it must influ- 

 ence to a certain extent the economy of the animal. Were 

 we, however, to employ it in higher divisions than generic 

 ones, some confusion would certainly arise. It would cause 

 the separation of many genera which are nearly allied, and 

 even divide several natural families. Thus, for example, 

 among the porcellaneous shells, it would separate the olives 

 from cones, the former being destitute of an operculum, 

 while the latter possess one. These two genera, however, 

 belong to a natural family, the animals of both genera hav- 

 ing a respiratory tube upon the head, and the eyes placed 

 on the sides of the tentacula, instead of being situated, as 

 in the other gasteropoda, on the tips or at the base. This 

 circumstance is calculated to convince us of the necessity 

 of caution in the admission of characters. These may at 

 first appear to be of extensive occurrence, and well adapted 

 for the formation of families, but unless they exercise some 

 visible influence on the animal, they can never be employ- 

 ed with propriety in a natural system, however convenient 

 they may be in an artificial arrangement. Operculated 

 shells may be considered as generically different from those 

 which are destitute of that organ, without any injury to the 

 natural method. It would even, in many instances, be con- 

 venient. 



Amongst univalve shells, considerable differences are ob- 

 servable in the shape and position of the cavity of the shell. 



