Bibliographical Notice. 445 



ence 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 stripped off. In order to 

 prevent its tubes being choked by an accumulation of the imported 

 material, the Pandora living between tide-marks gradually lengthens 

 that end of its shell. The variety which inhabits deeper water is 

 not exposed to fluctuations of this kind ; it therefore does not re- 

 quire any such provision, and lies undisturbed in its level bed. 

 This may explain the variation iu the proportion of length and 

 breadth which is exhibited by the two forms. The difference of 

 thickness in the shells of P. incequivalvis and its varieties also de- 

 pends on habitation. I am also inclined to think that, with regard 

 to every species living both in the Littoral and Coralline zones, the 

 shell is thicker in the former than in the latter. Examples to illus- 

 trate this proposition occur in Venus gallina and its varieties stria-^ 

 tula and laminosa, Mactra solida and its variety elliptica, Trochus 

 ziziphimis and its small conical variety, Buccinum undatum and its 

 variety zetlandica, and in many other species. Experiments made 

 by Dr. Davy, 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 in discriminating species from 

 varieties, of which the comparative soHdity and size are the sole or 

 chief criteria." 



Now, surely, there is a great deal assumed here. That conve- 

 nient little word "may" holds a very important place in this argu- 

 ment ; and Mr. 'Jeffreys supposes an instance of wonderfully rapid con- 

 formity to requirement when he urges that the posterior extremity 

 of the shell may be lengthened in the course of a few days to pre- 

 vent the tubes being choked by an accumulation of the shifting ma- 

 terial of the sea-shore. We would suggest that the animal might 

 crawl upwards out of the deepening sand somewhat more rapidly 

 than the shell would be likely to grow, and, moreover, that it must not 

 be forgotten that the size and form of the shell depend upon the size 

 and form of the animal contained within it. Again, it is undoubtedly 

 true that a species which has a considerable range in depth, will be 

 frequently found to have its shell much more strongly developed in 

 the Littoral and Laminarian zones than it is in greater depths of 

 water ; but this fact does not prove that a thin-shelled, deep-water 

 member of a genus is specifically identical with a stronger-shelled 

 form which invariably inhabits shallower water, and differs from it 

 not only in respect to the substance of the shell, but in many other 

 particulars of structure also ; moreover we are not aware that the 

 comparative tenuity of P. obtusa has ever been relied upon as of 

 weight in maintaining its specific character. Forbes and Hanley 

 clearly point out the characteristic differences of the two allied 



