XII 



PHYLUM MOLLUSCA 



703 



shell of the fresh-water Mussel, and lacking also its bilateral 

 symmetry. It is in the form of an elongated hollow cone closely 

 wound round a central axis. The apex of the cone is the organic 

 apex of the shell, corresponding to the umbo of the fresh-water 

 Mussel, and is the point from which the growth of the shell has 

 proceeded : the base is represented by the wide oblique opening 

 the mouth or peristome of the shell. Starting from the apex along 

 the internal cavity of the spirally -wound cone, in order to reach 



^ ^^ 



FIG. 018. Shell of Triton rubicundus. 



Natural size. 



FIG. 619. Longitudinal median section of the 

 shell of Triton rubicundus. 



the mouth in an adult shell, we pass completely round the central 

 axis five times i.e. the spiral consists of five turns. In following 

 the turns, the direction taken is to the right, that is to say, the 

 spiral of the shell is a right-handed or dextral one. The axis 

 (Fig. 619) is in the shape of a twisted shelly rod the columella 

 containing a narrow lumen ; it is formed by the close union of 

 the axial portions of the wall of the spiral. The windings of the 

 spiral are marked on the outer surface of the shell by a narrow 



