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Show 396 MR. E. A. SMITH ON SHELLS FROM ADEN. [June 16, The remaining eight species 1 are evidently correctly assigned :- 1. Chiton siculus. 2. ,, discrepans. 3. Philine aperta. 4. Lima injiata. 5. Area lactea. 6. Venerupis irus. 7. Petricola lithophaga. 8. Gastrochcena dubia. The subject of the relationship of the Faunas of the Mediterranean and Red Seas is most attractive, and has been more or less fully discussed by R. A. Philippi, Paul Fischer, R. MacAndrew, xi. Issel, and A. H . Cooke. Certain species have been regarded by some of these authors as common to the two seas, and it has been conjectured by them that an intermingling of the faunas of these seas has occurred in past ages when a junction of their waters apparently existed. Species which are commonly regarded as Mediterranean, and which occur in the Gulf of Suez, are supposed to have gradually migrated southward, and, when the two seas became separated, to have established themselves as permanent inhabitants of the warmer waters. Now, after a careful study of the geographical distribution of these species, finding that all exist also far east in the Indian Ocean, having a much greater range in this direction than through the Mediterranean and some distance into the Atlantic, and considering the Indo-Pacific character of the Red-Sea fauna, it seems to me equally or more reasonable to suppose that the Mediterranean specimens were derived from a Red-Sea source than vice versa. It may be urged in opposition to this theory, how is it that such and such species have been found at Suez only, and at no other part of the Red Sea? The answer to this is simply, that the shores of the Red Sea have only been cursorily examined in a few places, and I fully anticipate that, whenever other more southern parts have been as well investigated as the Gulf of Suez, most of these species will be met with. Already two out of the eight have been recorded as far south as Assab. Geographical distribution of species is such an enigma in many cases that one feels reluctance in launching forth any theory whatever. Some species, as far as our present knowledge of them extends, appear to have an almost unlimited range; whilst, on the contrary, other allied forms seem to be equally restricted. As examples, I may instance Area lactea and A. olivacea. The former little species ranges through the Mediterranean into the Atlantic as far north as this country, southward along the West Coast of Africa past the Atlantic Islands to Ascension Island, on to the Cape of- 1 The estuarine forms Cerithium mammillatum and Leuconia denticulata are not taken into account, as the subject under consideration is the relationship of the Marine faunas of the two seas. Vide remarks upon the former previously given. |