Page:Memoirs of the Geological Survey of Great Britain, Volume 1.djvu/390

Rh general, littoral, and indicating a much less depth of water than existed previously during the deposition of the marls. The abundance of Purpura lapillus, and the presence of Littorina littorea, may be mentioned as especially characteristic of the shelly gravels which in Wexford have been found by Captain James to contain numerous specimens of the reversed variety of the Fusus antiquus, known under the name of Fusus contrarius, and common in the red crag. At present the reversed form is as rare among specimens of that Fusus, as the dextral form was anciently. It is difficult to conjecture a sufficient cause for the prevalence of the monstrous over the normal form during two geological epochs. The discovery by Captain James of Turritella incrassata (a crag fossil), of a southern form of Fusus, and of a Mitra, allied to a Spanish species, in these southern Irish beds, associated with the usual glacial species, is an important fact, suggesting the probability of a communication southwards of the glacial sea with a sea inhabited by a fauna more southern in character than that now existing in the neighbourhood of the region where these relics were found. It is a fact which will bear strongly on the question of the point of time, whether before, during, or after the glacial epoch, certain freshwater beds containing important fossils, remains of vertebrata and mollusca, were deposited. It also bears importantly on the general question of the distribution of climates on this side of the Atlantic during the later tertiary periods. At present we have an intermediate (the Celtic) marine fauna, separating the Boreal and South-European or Lusitanian types. But that such an intermediate type need not have existed at a time when the Boreal fauna ranged, almost exclusively, farther south than now, we may convince ourselves by looking to the state of the marine fauna at the opposite coasts of the Atlantic. In latitude 42° N., we find a cape of no great prominence, and of recent geological origin, constituting the barrier, or rather marking the line of demarcation between a fauna of character in great part as northern as that which prevailed in our seas during the glacial period, and one of fully as southern a character (if not more so) as that now prevailing on the coasts of Portugal. For a short space, and but a very short space, the two faunas intermingle; but there is no distinct intermediate marine fauna like that now widely separating the northern and southern European types. The best evidence I can quote on this subject is that of Dr. Gould, whose State Report on the Invertebrata of Massachusetts, is a most carefully elaborated and excellent work. In summing up his results, he notices the collision of the two types of marine fauna alluded to as follows: "Cape Cod, the right arm of the commonwealth, reaches out into the ocean some fifty or sixty miles. It is nowhere many miles wide; but this narrow point of land has hitherto