The northern and western relations of the glacial testacea are very remarkable. Not only are several of the characteristic species, as we have already seen, identical with forms now known only in Arctic Seas, or on the coasts of Boreal America, but fully a third of the entire assemblage are species still existing in the American Seas, and at the same time on the coasts of Europe. At the present day there are sixty-six species of testaceous mollusks common to the coasts of the United States, north of Cape Cod, and Europe. Of these sixty-six species, not one has its northern European limit south of Britain, and only ten (excluding two pelagic forms) range to the seas of Southern Europe, and of these some are doubtful identifications. On the other hand, no fewer than forty-five are recorded inhabitants of the Arctic Seas,—and probably many more live there, for I have no lists by which to institute a comparison of the British fauna with that of Lapland, or even Iceland. In the following table the first column contains the names of testacea common to Europe and North America, as recorded by American authors of repute, especially Dr. Gould, who took every pains to identify his species correctly with their European analogues. Some few of these are doubtful; and it will be observed, that the doubtful species are just those deficient in the middle column, exhibiting the localities in which the species have been found fossil in beds of the glacial epoch. In the third column the European and Arctic ranges of the species are given, This table shows that no fewer than fifty-one out of the sixty-five are known as glacial fossils. Of the remaining fourteen, two, viz., Spirula Peronii and Janthina fragilis are pelagic mollusks wandering from the south, and with them may be classed Teredo navalis, carried about in floating wood. Two, Kellia rubra and the Skenea, are minute species living in stony ground near highwater-mark, and not likely to be preserved fossils; whilst the three Chitons, although larger from inhabiting similar situations, and being extremely fragile, falling to pieces after death, are in the same category. The Modiola glandula (Crenella decussata of English conchologists) is also a minute shell. Modiola discrepans and discors are doubtful identifications. The remainder, including Buccinum Donovani, and two species of Margarita, may yet be expected to occur in the drift.
From this table we may fairly infer, that the identity, so far as it exists, between the testaceous fauna of Boreal America and that of Europe, was established, at latest, during the glacial epoch; and the occurrence of some American forms in the crag, as we have previously mentioned, would show that it commenced prior to that epoch. It is very important to observe that the ancient relationship between the marine mollusks of the new and old world is not maintained by pelagic species or floaters—the few of those in the list are recent introductions
