18 INTRODUCTORY 
this is to be determined by the oblique direction of the aperture 
to the under part, where the lip rarely extends so far as on the 
upper part. In fixed shells, such as Serpule, there is no difficulty, 
as the side which is sessile must be considered as the base or 
under part. ‘Thus, in the Serpula Lucida the fixed part is some- 
times very small, and the mouth turns spirally upwards, in a 
contrary direction to the sun ; and therefore must be considered a 
reversed or heterostrophe shell, the same as if the volutions nearest 
the mouth had turned laterally upon the centre or fixed ones. 
This shell, indeed, is most frequently found with regular lateral 
volutions; and though subject to great variety, with respect to 
contortions, it invariably turns the aperture one way. 
In some species of Nautilus, however, there can be no rule to 
ascertain whether the shells are dextral or sinistral; for when the 
aperture is exactly central, the lip embraces the body equally, and 
the sides of the shell are similar. 
Chambers are the cavities divided by partitions, at regular or 
irregular intervals; as in the Spirula.s Plate III. fig. 11, WW 
WW yw. 
In some of the Serpule there are also divisions, but they are 
not regular asin the Nautili and Spirule ; and besides, they differ 
from them in being devoid of a siphunculus or communication 
between the chambers, the animal forms a complete partition, and 
adds to its shell, which it would appear to be necessitated to do 
from its body growing too large for its abode. 
Several of the Patelle have chambers formed of laminous par- 
titions, subspiral cells, or processes; these in general lie horizon- 
tally, and are quite open at one end, as in the Patella Testudinaria, 
the Crepidula, and Calyptrea. 
Umbilicus is in general a circular perforation in the base of 
the lower whorl, or body, of many univalve shells. This is com- 
mon to most of the Trochi, in some species of which it penetrates 
from the base to the apex; widest at the base, and gradually~ 
tapering to the top. Plate III. fig. 1, 2. 
Sub-umbilicated shells are those which have the umbilicus 
covered in a greater or less degree by a thin process; which, in 
some, almost entirely closes the opening or mouth. This charac- 
