MOLLUSCA. 5] 
In the genus Argonauta (pl. 76, fig. 17) the ends of two of the arms are 
greatly dilated, and these dilations clasp the shell upon each side, which 
seems to be formed by a secretion from their inner surface. 
In some of the naked land-snails, like p/. 77, fig. 21, a small thin shell 
may be taken out of the back, and the cuttle-fish has a corresponding 
internal shell, known as cuttle-bone, which is more complex, having a hard, 
rough surface above, and a series of close-set, thin, parallel plates beneath. 
The lower end terminates in a point which corresponds to those fossil 
organic remains of cephalopoda, named Belemnites. 
In bivalve shells the mantle is slightly attached to the shell, in some 
degree parallel with its margin, and at a little distance from it; but this line 
of attachment, which is named the pallial impression, varies considerably 
in its course in different families, and through this variation affords 
distinctive characters. The mantle, or projections from it, secretes the 
spines, rugosities, and other appendages, by which the shell is varied. 
Sometimes the deposition of the shell goes on smoothly, when a thickening 
of the margin, a varix, or a row of spines will be secreted, to be followed 
by a smooth space, and this alternation takes place as long as the animal 
grows. See pl. 75, jigs. 101-104, 111,119. Univalve shells are usually 
strengthened by having the margin of the aperture thickened, and as this 
is added from time to time, the shell may acquire a ribbed appearance, as 
in figs. 101, 102. In other cases, as in fig. 91, the old lip is absorbed 
before a new growth is started, so that the shell remains smooth. 
Sometimes the margin is not formed until the animal attains its full growth. 
The oblique deposition of calcareous matter in spiral univalves gives 
them an elongated form, as in pl. 75, fig. 117, and when less oblique, the 
shell is more robust (fig. 98). In forms like Patella (fig. 77) the calcareous 
deposit extends in equal degrees; but if the increase is more abundant upon 
one side, the spine is curved, as in fig. 81. When the aperture is lateral, 
and the shell has little or no obliquity, the forms seen in pl. 76, figs. 2, 4, 
7, 11, are the result. 
In spirivalve shells the solid axis is named the columella, and a muscle is 
attached to it which connects the animal with the shell, and enables it to 
retire within it. .The columella is often marked with prominent folds and 
tooth-like projections, which afford generic characters. The anterior 
extremity of the mantle is elongated in some genera, and the edges brought 
in contact, so as to form a slender tube, through which water passes to the 
gills; and as this siphon secretes calcareous matter, the shell takes the 
rostrated form, as in pl. 75, figs. 109-112. 
Spiral shells are usually dextral, increasing towards the right, in which 
they resemble an ordinary screw; but there are a few genera, exceptional 
species, and varieties of dextral species, which are sinistral (pl. 75, fig. 100). 
A bivalve shell is considered sinistral when the projecting points at the 
hinge, named teeth (which present a certain degree of uniformity in each 
species), are changed so that the modification which belongs to the one 
side normally is found upon the other. 
Some of the spirivalves, when the animal retires into the shell, as ‘in fig. 
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