THE SHELL. 41 



American oyster, according to Prof. Wm. B. Rogers, contains: 

 Carbonate of lime, 95"18 per cent.; phosphate of lime, 1*88 per 

 cent.; silex, '40 per cent.; water, 1*62 per cent.; animal matter, 

 •45 per cent. 



Shells are perceptibly harder than, and will scratch calc-spar. 

 Their specific gravity is somewhat higher than that of Carrara 

 marble, being abont 2*75 to 2"85 for the prosobranchiates, and 

 varying but slightly in the pulmonates. 



With reference to the coloring of shells (terrestrial as well as 

 marine species) Fischer points out that as a rule, brilliancy of 

 coloring increases towards the equator. He suggests three 

 zones of coloration corresponding to the thermal zones, which 

 may be designated as : 



1. Monochromic or frigid zone. 



2. Oligochromic or temperate zone, 



3. Polychromic 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 manj'- 

 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 Protective Coloration. A number of papers on mimicry or 

 adaptive resemblance as a means of protection of mollusks 

 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 

 frequentl}^ remarked, but it is probably not so often for purposes 

 of attack or security as the result of the food upon which they 

 live or a dj^e obtained by contact with it. Mr. Morse gives a 

 number of instances of coincidence of color between American 

 marine mollusks and their surroundings which he supposes to 

 be evidences of protective adaptation f of these, three principal 

 species of Crepidula or slipper-limpet, may be cited : G.fornicata 

 is drab, variously rayed and mottled with brown, and it liA^es 

 attached to stones or on the outside of shells of similar color ; 

 C. convexa has a dark brown shell, and is found on Ilyanassa 

 obsoleta or on the roots of sea-weed; G. plana is white, and 

 inhabits the inside of the aperture of dead shells, which is also 

 white. Mr. Melvill mentions that Ovulum uniplicatum^ as it is 

 found on the Florida coast, is yellow or purple according to the 

 color of the gorgonia on which it occurs. 



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

 surface of shells appear to depend, in a great measure, on the 



* Dr. P.Fischer, Jour, de Gonch., xxiii, 105, 1875. 



t E. S. Morse, Proc. Bost. Soc. Nat. Hist., xiv, 141, 1871. 



