1891.] MR. E. A. SMITH ON SHELLS FROM ADEN. 397 



Good Hope and Natal, and finally it is known from the Red Sea and 

 Philippine Islands. The other species, A. olivacea, the distribution 

 of which, as far as we know, is as limited as that oi A. lactea is 

 extensive, has at present only been recorded from the Philippines. 

 I could multiply cases of this kind, but the one mentioned is 

 sufficient to demonstrate the unaccountable difference in the dis- 

 tribution of allied forms. There seems to be an unfathomable 

 something in their nature which permits the one to live under very 

 varied conditions, in temperatures greatly differing, and in waters 

 of which the chemical composition is dissimilar, and on the other 

 hand which does not allow the other to exist excepting under special 

 and limited conditions. It is so in the vegetable kingdom. Do we 

 not find some plants which will grow almost anywhere, in all kinds of 

 soil, whereas to others existence appears to be possible only amid 

 very special surroundings ? Being cognisant of such facts as these, 

 it is with much diffidence that I have suggested the migration, so to 

 speak, of the species in question, or some of them at least, from the 

 Red Sea into the Mediterranean. However, taking all points into 

 consideration, I think this supposition is likely to be as correct as the 

 view usually entertained. 



Some support to this theory is derived from a study of the 

 emigration of species from the Red Sea to the Mediterranean and 

 vice versa since the opening of the Suez Canal. From the reports 

 upon this subject by Fuchs ^ Keller ^ Krukenherg% and others, it is 

 evident that there is a greater pilgrimage taking place northward 

 than towards the south, and this, to some extent, is possibly attri- 

 butable to the movement of the current from the Red Sea to the 

 Bitter Lakes being faster than that from the Mediterranean southward, 

 for there is a flow in both directions, owing to the great evaporation 

 in the Bitter Lakes. At present two Red Sea forms, Mytilus variabilis 

 and Mactra olorina, have been taken living at or near Port Said ; on 

 the contrary, no Mediterranean species has as yet got through to 

 Suez, but Cardium edule (if correctly identified) is said to have 

 almost reached there. Although certain species may extend north- 

 ward and to the south, it yet remains to be seen if they become 

 modified to any extent, supposing the altered temperature and 

 chemical composition of the water into which they may have 

 migrated permit their race to be perpetuated. 



I can well imagine that eventually it will be found that all the 

 rest of the species have as wide and very nearly the same distribution 

 as Area lactea, and therefore the possibility is suggested that their 

 presence in the Mediterranean may have originated from the 

 Atlantic end and not from the eastern or Red Sea extremity. 

 Suggestive of this is the fact that specimens of the same species from 

 the Atlantic Islands (Madeira, Canaries &c.) and the Mediterranean 

 are absolutely identical, whereas, in some instances at all events, in 

 the Red Sea equivalents some slight modifications are noticeable. 



^ Die geologische Beschaffenheit der Laudenge von Suez. Wien, 18774 

 ^ Neue Denksehrift. allgem. Schweiz. Gesellsch. 1883, vol. xxviii. pt. 3. 

 ^ Vergl.-physiolog. Studien, 1888, 2nd ser., 5th part, 1st half. 



Proc. Zool. Soc— 1891, No. XXVII. 27 



