Page:Memoirs of the Geological Survey of Great Britain, Volume 1.djvu/380

Rh of the Clyde district, in company with, and under the guidance of Mr. Smith; in Wexford and Wicklow with Sir Henry De la Beche and Captain James, and alone in the Isle of Man, Cheshire, Lancashire, Anglesea, the north-east of England, the Forth district, and Caithness. For the statements herein made, I therefore hold myself responsible, and put them forth as the results of personal observation.

The total number of species of mollusca found in these beds in the British Isles, is about one hundred and twenty-four. With a few exceptions, they are all forms now existing in the British Seas. Nevertheless, they indicate a very different state of things from that now prevailing. As a whole, this fauna is very unprolific, both as to species and individuals, when compared with the preceding molluscan fauna of the red and coralline crags, or that now inhabiting our seas and shores. This comparative deficiency depends not on an imperfect state of our knowledge of the fossils in the glacial formations—on that point we now have ample evidence—but on some difference in the climatal conditions prevailing when those beds were deposited. Such a deficiency in species and individuals of the testaceous forms of mollusca, indicates, to the marine-zoologist, the probability of a state of climate colder than that prevailing in the same area at present. Thus the existing fauna of the Arctic Seas includes a much smaller number of testaceous mollusks than those of mid-European Seas, and the number of testacea in the latter is much less than in south-European and Mediterranean regions. It is not the latitude, but the temperature which determines these differences. Hence we find the number of testacea inhabiting the east coast of America, between the parallels of 45° and 50° N., where the waters are climatally influenced by the currents from the Arctic Seas, ia much less than that assembled several degrees farther north on the coasts of Europe. In the following table, the number of testaceous mollusks of four well-ascertained faunas is contrasted with a summary of the testacea found fossil in the glacial formations of the British Isles. From it, it will be seen that the glacial molluscous fauna holds a numerical rank between