20 THE SHELL. 



zones of coloration corresponding to the thermal zones, which 

 may be designated as : 



1. Monochromic or frigid zone. 



2. Oligochromic or temperate zone. 



3. Polychroniic or tropical zone. 



Of course exceptions are numerous modifications based upon 

 habits, ocean currents, adaptive coloration, etc.. M. Fischer 

 particularly cites the melanism which characterizes so many 

 shells of the West Coast of North and South America, giving, 

 among other examples, the numerous species of sad colored and 

 lugubriously named Trochi, which pervade those regions.* 



On adaptive coloration. A number of papers on mimicry or 

 adaptive resemblance as a means of protection of animals 

 against their enemies have been published, and the subject has 

 awakened much interest and discussion. Coincidence of colora- 

 tion of molluscous animals with their surroundings has been 

 frequently remarked, but it is probably not so often for purposes 

 of attack or security as the result of the food upon which the^y 

 live or a dye obtained by contact with it. I have shown in my 

 volume on the Cephalopoda that conscious adaptation of color 

 has been noticed in the Octopus. Mr. Morse gives a number of 

 instances of coincidence of color between American marine mol- 

 lusks and their surroundings which he supposes to be evidences 

 of protective adaptation.f 



" The thickness, the roughness, and the smoothness of the sur- 

 faces of shells appear to depend, in a great measure, on the still- 

 ness 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 dis- 

 tinct species. Thus Murex anguliferus is merely a Murex ramo- 

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

 subcarinatus, M. cinguliferus, M. tarentinus, and M. polygonus 

 are all varieties of one species. Murex magellanicus, when found 



* Dr. P. Fischer, Jour, de Conch., xxiii, IQ.% 1875. 



f E. S. Morse, Proc. Bost. Soc. Nat. Hut., xiv, 141, 1871. 



