Page:A Treatise on Geology, volume 1.djvu/356

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Serpentine appears actually to prevent the abundance of shelled mollusca. The character of the water, the proximity of river-mouths, the set of currents, are all elements of local distribution; but after allowance is made for all these circumstances, depth in the water remains to be considered.

In the following table the numerical distribution of shells in the various regions of depth of the Ægean is expressed.

To all the eight regions only two species of mollusca are common, viz. area lactæa, cerithium lima. Nine are found in six regions, seventeen in five regions. The observed distribution of other species which occur in more than one region agrees with a general inference that the extent of range of a species in depth is proportioned to the extent of its geographical distribution.