66 : MOLLUSCA. 
sent state of the science, and may be permitted on that ac- 
count; but in proportion as our knowledge of the mollusca 
advances, this distinction will be deemed inexpedient. In. 
deed, were this division adopted, the genera Argonauta and 
Nautilus would be torn from each other, although they are, 
by Cuvier and many others, regarded as members of a fam- 
ily of cephalopodous mollusca. The Nautilus lacustris of 
Lightfoot, now constituting the genus SEGMENTINA, would, 
in that case, likewise be separated from the genus Planor- 
bis, with which it is very closely allied. In the meantime, 
until our knowledge of the multilocular testacea arrives at 
a greater degree of perfection, such divisions may be em- 
ployed as convenient, and of easy application. 
The preceding remarks apply to those shells which belong 
to the cephalous mollusca. Among the bivalve shells, 
which beiong to the acephalous mollusca, the characters 
which. they exhibit are of very different degrees of import- 
ance. Here, as among the univalves, the appearance of the 
shell enables us to form an idea of the organization of the 
animal, so that the characters thus furnished by the shell 
may be safely employed in a natural system. 
The bivalve shells, in general, possess the faculty of mov- 
ing from one place to another, or of attaching themselves 
to rocks and stones, by means of temporary threads. These 
are termed free shells. But there are others which secrete 
at their birth a calcareous cement, which unites the shell 
to the rock or stone immoveably for life. These last are 
known by the name of fiwed shells. If we thus consider 
the difference in the economy of these two divisions of bi- 
valves, we may reasonably expect to find corresponding dif- 
ferences in their organization. The free shells contain ani- 
mals endowed with locomotion, and by consequence with 
