UNIVALVE MOLLUSKS 109 
The upper part of the shell is known as the spire, 
marked sp, of which the very top, a is the apex, 
while each turn of the spire is a whorl, the largest 
of which, b. w., is the body whorl. The spiral 
groove between the whorls is the suture, marked 
s. The central post, c, is the columella, while the 
central opening, uw, is the umbilicus. The little 
open tube marked ca is called the canal, and the 
outer edge of the aperture, o. J. is the outer lip. 
The inner lip, in this specimen, is grown to the 
columella and does not appear, though in some 
shells, as shown in Fig. 194, it is very evident. 
Many shells have no canal, and in most instances 
we judge that the animal which has such a shell 
is herbivarous, while those with canals are car- 
nivorous. Line of growth run parallel to the edge 
of the outer lip, while spiral lines run around the 
shell and cut the lines of growth. Varices are 
enlarged portions of the shell, parallel to the lines 
of growth, while the operculum. is a kind of door 
that the animal pulls up to the aperture, after he 
has withdrawn into his shell. With these few sim- 
ple definitions, we are ready to describe almost any 
of the numerous shells that we are to consider in 
the remaining part of the book. The name of the 
species whose shell is shown in Figure 94 is Chry- 
sodomus dirus, Rve., the Dark Chrysodome. The 
shell is of a dark liver color, though this is often 
obscured by an ashy powder, giving it a dingy 
appearance. 
As we have considered this type of shell out of 
its natural order, we will now turn our attention 
to what is properly the first species of the great 
