Page:Memoirs of the Geological Survey of Great Britain, Volume 1.djvu/401

388 Celtic fauna, and the species are such as are assembled together far to the north on the coast of Norway. Among them are many of the most remarkable of the forms, found fossil, in the glacial beds, as Cemoria noachina, Trichotropis borealis, Natica groenlandica, Astarte elliptica, Nucula pygmæa. These are associated with Terebratula caput serpentis, Crania norvegica, Emarginula crassa, Lottia fulva, Pecten danicus, Neæra cuspidata, costata and abbreviata, and many peculiar echinoderms and zoophytes, which are either species known only far north, or found only at considerable depths in localities farther to the south.

As these northern "outliers"—a term which well expresses their character—occur in districts which are remarkable for the number and extent they present of upheaved glacial sea-bottoms, their depth and contents have suggested the following explanation:—

When the bed of the glacial sea was upheaved, that upheaval, as we have already seen, raised above water only such portions of it as had been formed in a moderate depth. Such tracts of that sea as were of moderate depth, and consequently inhabited by a peculiar fauna, would still remain below water, though changed in level. A portion of their fauna,—whose organization might be too delicate to endure the sudden change of conditions,—would be destroyed; but another portion, consisting of such species as had greater capacities for vertical range, would survive; for in the deeper parts of the seas of our area, conditions of temperature would still remain such as were required by these isolated northern forms. Let, of the following diagrams, represent the conformation of a part of the British Seas during the Glacial epoch, throughout their area, of which a boreal or subarctic fauna prevailed; at the close of that epoch the elevation of the sea-bed, whilst it converted into dry land its shallower portions, left the deeper tracts (which within our area were few in number and small in extent) still under water (diagram ). In these depths the arctic forms would still live on, whilst climatal changes would so alter the zoological character of the shallows of our seas (as represented in the unshaded portions of water in figure ), as to isolate the assemblages of animals in the depths, and leave them, as it were, in the condition of northern outliers.



This isolation of the northern marine animals, and restriction of them