SHELL. 21 



in smooth water, is covered with large acute foliaceous expan- 

 sions ; but the same shell living in rough seas is without any 

 such expansions, and only cancellately ribbed. In such situa- 

 tions it seldom grows to a large size ; but when it does so, it 

 becomes very solid, and loses almost all appearance of cancella- 

 tion. Triton maculosus is very widely spread over the ocean in 

 different temperatures and different kind of seas ; it consequently 

 offers a multitude of varieties both of size and surface, all gradu- 

 ally passing into each other, and most probably produced by the 

 operation of the foregoing causes. Indeed, a vast number of 

 merely nominal species have been formed from the habit, too 

 prevalent among conchologists, of describing from single speci- 

 mens, or even from several individuals brought from the same 

 locality, which would never have been considered as distinct 

 had collectors kept in their cabinets a series of specimens found 

 under different circumstances, and studied, on the coasts where 

 they are found, the variations which shells undergo."* 



Shell. 



Rest periods in the growth of mollusks are sometimes, as in 

 the Muriees, marked by a thickening of the edge of the shell, 

 caused by continuous depositions of shell material, forming a 

 ridge or varix ; and the rate of growth may be thus traced readily, 

 in numerous mollusks. Thus in Murex one group shows three 

 varices upon each whorl, indicating that a period of three years, 

 or at least three seasons of activity is required for the completion 

 of a single whorl of growth. In another group of Murices the 

 varices are more numerous, as many as four to ten being counted 

 on a whorl. In Triton, the varices are two on each whorl, but 

 nearly alternately situated, so that the varices of each whorl oc- 

 cupy an intermediate position to those on the preceding whorl : 

 in Ranella there are also two varices but they form a continuous 

 fringe or wing-like expansion on each side of the spire, showing 

 a very regular growth by periods of half-whorls. 



The accretion of surface during growth is not continuous but 

 is made by minute layers, around the margin of the aperture, 

 each extending a little beyond its predecessor, and the edges of 



* Dr. J. E. Gray. Philos. Trans., 771, 1833. 



