722 



ZOOLOGY 



SECT. 



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 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. 609) is in the 

 shape of a twisted shelly 

 rod — the columella — con- 

 taining 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 im- 

 pressed spiral line or suture, 

 parallel with which are 

 numerous fine ridges and 

 depressions — the lines of 

 growth ; the increase in size 

 of the shell takes place in 

 the direction of these lines, 

 not at right angles to them 

 as in the shell of the fresh- 

 water Mussel, and the lines 

 that more strictly correspond 

 to the lines of growth of the 

 latter are excessively fine 

 striae which run transversely 

 to the stronger lines. At 

 certain points, usually three 

 in a full-grown shell, the 

 spiral is interrupted by a transversely directed edge which appears 

 to overlap the succeeding portion ; this edge marks the position 

 which the mouth of the shell has occupied during regularly 

 recurring periods of arrest of growth, probably annual. 



The mouth of the shell is bordered on the side turned away 

 from the columella by a prominent rim or outer lip of the 

 peristome ; this is produced at the extremity farthest from the apex 

 of the shell into a spout-like process — the siphonal process. The 

 prominent edge of the peristome is in relation to the dorsal 



Firs. 60S.— Shell of Triton nodiferus. 



Natural size. 



