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 
Stulidia, 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 Lymnza, 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 wnilo- 
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 distine- 
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- 
