42 THE SHELL. 



stillness or agitated state of the water in which they reside. Shells 

 which have branching or expanded varices, like the Murices, are 

 also much influenced by circumstances, and hence many mere 

 varieties, arising from local causes, have been considered as 

 distinct species. Thus Murex anguliferus is merely a Murex 

 ramosus with simple varices ; and Murex erinaceus, M. torosus, 

 M. subcarinatus , M. cinguliferus, M. Tarentinus^ and M. poly- 

 gonus are all varieties of one species. Murex Magellanicus, when 

 found in smooth water, is covered with large acute foliaceous 

 expansions; but the same shell living in rough seas is without 

 any such expansions, and only cancellately ribbed. In such 

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

 it becomes very solid, and loses almost all appearance of cancel- 

 lation. Triton maculosus is very widely spread over the ocean 

 in different temperatures and different kinds of seas ; it conse- 

 quently offers a multitude of varieties both of size and surface, 

 all gradually 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." — Dr. J. E. Gray, 

 Philos. Trans., Ill, 1833. 



I have inserted the above as a sort of warning to species-makers 

 and because my own experience partially confirms it ; but Dr. 

 Gray was too apt to propose theories in explanation of difficulties 

 and to find illustrations and confirmations of such theories in 

 hasty assumptions rather than by the careful observation of 

 facts. 



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

 the Murices, 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 (xliii, 5), 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 (xliii, 

 7), as many as four to ten being counted on a whorl. In Triton, 

 the varices are two on each whorl, but nearly alternately situated 

 (xlvi, 54) so that the varices of each whorl occupy an interme- 

 diate position to those on the preceding whorl ; in Ranella there 

 are also two varices but thej^ form a continuous fringe or wing- 



