Page:Memoirs of the Geological Survey of Great Britain, Volume 1.djvu/383

370 formerly termed Purpura incrassata. It is also of consequence to note the fact that the species most abundant, and generally diffused in the drift are essentially northern forms such as Astarte elliptica, compressa, and borealis, Cyprina communis, Leda rostrata and minuta, Tellina calcarea, Modiola vulgaris, Fusus bamfius and scalariformis, Littorinæ and Lacunæ, Natica clausa and Buccinum undatum; and even Saxicava rugosa and Turritella terebra, though widely distributed, are much more characteristic of north European than of southern seas.

I have elsewhere shown that among marine animals, zones of depth correspond to parallels of latitude, even as the latter are, in like manner, represented on land by zones of elevation. When I made known the facts proving this law, I did not distinguish between the law as really maintained by representative forms, and as apparently exemplified by identical species. This distinction I have pointed out in this essay when treating of alpine plants. It will be seen by and by that it is equally important to bear it in mind, in the case of the boreal forms of marine animals found at great depths in southern seas. The truth of the law itself, however, holds good, and the knowledge of its existence naturally leads us to inquire how far the apparently boreal or arctic fauna of the glacial beds might owe their climatal character to such a cause. This is an important question, for unless we can determine positively that depth was not the element to which the peculiar facies and numerical weakness of this submarine fauna were due, our climatal determinations, so far as they depend on zoological data, become mere hypotheses.

Fortunately, however, among the species enumerated are several which ought to afford us a certain clue to this matter. Such are the Littorinæ, the Purpura, the Patella, and the Lacunæ, genera and species definitely indicating, not merely shallow water, but in the three first cases a coast line. Were these shells only found among the disturbed and amorphous beds of drift, they would scarcely serve as evidence on so nice a point, since they might have been transported; but they occur also in the undisturbed fossiliferous clays of this formation, associated with bivalve and other moUusca of delicate conformation and in a state which certainly indicates that they lived and died on the spots where they now are found. This is especially the case among the Clyde deposits. A most important fact, too, is that among the species of Littorina, a genus, all the forms of which live only at water-mark, or between tides, is the Littorina expansa, one of the forms now extinct in the British, but still surviving in the Arctic Seas.

To make this evidence more clear, it is necessary briefly to notice the vertical distribution, or distribution in depth of existing mollusca in the British Seas.