MOLLUSCA. 65 



In some cases this cavity is simply conical, while in others 

 it is conico-tubular, either revolving horizontally round a 

 centre, or spirally twisted upon an axis or pillar. These 

 circumstances furnish characters of great importance in an 

 artificial system, as by means of them all testaceous bodies 

 may be arranged into two tribes, the one possessing a pillar, 

 round which the tube of the shell is twisted, while the other 

 is destitute of any pillar. The former have been termed 

 Sfulidia, the latter Astulidia. As a natural character, 

 however, these distinctions are of inferior importance, and, 

 if employed would occasion a separation between the genera 

 Planorbis and Lymnaea, which are demonstrated by Cuvier 

 to be nearly related. In the formation of genera, it may 

 be employed with advantage, even in a natural system, aided 

 by the structure of the pillar, and the direction of the whorl. 

 The last character which we have to notice while speak- 

 ing of the univalves, depends on the circumstance of the 

 cavity being entire, or divided into chambers, being unilo- 

 cular or multilocular. In the multilocular testacea there 

 are a number of transverse plates, in some species perfor- 

 ated, in others entire, which cross the cavity of the shell, 

 and, in general, divide the external cavity, in which the 

 animal resides, from the older and smaller ones, from which 

 it has receded. In an artificial arrangement, such distinc- 

 tions may. be employed with advantage, even in the forma- 

 tion of the primary divisions, but we entertain doubts as to 

 the propriety of using them in a strictly natural method. 

 We are in a great measure ignorant of the animals which 

 inhabit the multilocular shells, yet as far as our knowledge 

 goes, we are induced to regard the distinction as merely 

 conventional, and as unconnected with any peculiar order 

 of organization. Such a division may be useful in the pre- 



