In the preceding observations on the marine zoological phenomena, presented by the drift, I have strictly confined myself to evidence derived from beds of the same age, and universally recognised as typically belonging to the "northern drift," properly so called, i.e., formations of the Glacial epoch. There are, however, two English formations containing marine shells, many of them characteristic glacial species, the synchronism of which, with the glacial beds of the north-east of England, of Scotland, and Ireland, has not been granted, or has been received with a doubt These are the beds at Bridlington and the marine parts of the mammaliferous crag.
The discovery in the Irish drift of several of the most characteristic testacea of the mammaliferous crag, not elsewhere observed in glacial beds, has thrown a new light on the nature of both the English deposits of doubtful age, and enables us, without much misgiving, to refer them to the glacial epoch, and to regard them as probably marking its commencement before the severer climatal conditions had set in. We have seen already that m the red crag certain boreal species appear. In the mammaliferous crag the number of these increases, and the number of the southern forms materially decreases. The following analysis of the valuable catalogue of mammaliferous crag shells, included in the lists of Mr. Searles Wood, will show the state of our testaceous fauna on the eastern province of the British region during the period when that formation was deposited.
They consist, 1st, of species now living in the British Seas, and found fossil in true glacial beds. Those marked with an asterisk are elsewhere in contemporaneous beds, either confined to, or chiefly found in, the Irish drift.
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*Pholas crispata.
*Solen siliqua. Mya arenaria. Lutraria Listeri. Mactra solida. Abra Boysii. Corbula nucleus. Saxicava rugosa. Tellina solidula. *Donaz trunculus. Astarte borealis. Astarte compressa. Cyprina islandica. Venus fasciata. Cardium edule. Pectunculus pilosus.
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Nucula tenuis.
Mytilus edulis? (antiquorum). Pecten opercularis. Pecten obsoletus. Velutina lævigata. Littorina littorea. Turritella terebra. Purpura lapillus. Murex erinaceus. Fusus antiquus, and var.* contrarius. Pleurotoma rufa. Pleurotoma turricula. Natica monilifera? (catenoides of Wood).
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