PANDORA. 27 



difference of locality. When the littoral zone is sandy, 

 the surface is apt to be disturbed by waves and occasional 

 storms, so that the stratum may be of a greater or less 

 thickness at one time than at another : now it is covered 

 by a deposit of material thrown up by the sea ; in a few 

 days this cover may be stript off. In order to prevent its 

 tubes being choked by an accumulation of the imported 

 material, the Pandora living between tide-marks gradu- 

 ally lengthens that end of its shell. The variety which 

 inhabits deeper water is not exposed to fluctuations of 

 this kind ; it therefore does not require any such pro- 

 vision, and lies undisturbed in its level bed. This may 

 explain the variation in the proportions of length and 

 ])readth which is exhibited by the two forms. The dif- 

 ference of thickness in the shells of P. incequivalvis and 

 its varieties also depends on habitation. I am inclined 

 to think that, with regard to every species living ])oth in 

 the littoral and coralline zones, the shell is thicker in 

 the former and thinner in the latter. Examples to 

 illustrate this proposition occur in Venus gallina and its 

 varieties striatula and laminosttj Mactra solida and its 

 variety elliptica, Trochus zizij)hinus and its small conical 

 variety, Buccinum undatum and its variety Zetlandica, 

 and in many other species. Experiments made by Dr. 

 Da\y, Forchhammer, and BischofF have proved that the 

 quantity of carbonate of lime held in solution by sea- 

 water, and from which shells are produced, is greater on 

 the coast than in the ocean ; it is derived from the land, 

 and brought down to the sea by rivers and streams, the 

 washings of rain, and the action of waves. This fact 

 ought not to be lost sight of in discriminating species 

 from varieties of which the comparative solidity and 

 size are the sole or chief criteria. 



Lamarck at first named this species P. margaritacea, 



c2 



