396 MR. E. A. SMITH ON SHELLS FROM ADEN. [JuilC ! (5, 



The remaining eight syjecies^ are eviJently correctly assigned : — 



1 . Chiton siculus. 



2. ,, discrepans. 



3. Pliiline aperta. 



4. Lima injlata. 



5. Area lactea. 



6. Venerupis irus. 



7. Petricola lithophaga. 



8. Gastrocficena diibia. 



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, A. 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 south- 

 ward, 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 unhmited 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. 



