MOLLUSCA. 51 



In the genus Argonauta (^Z. Iß^fig. 17) the ends of two of the arms are 

 2;reatly 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 j^;?. 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, ])arallel 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 pi. 75, figs. 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 pi. 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 pi. 70, 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 pi. 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 {pi. 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.' 



255 



