COOKE : ON GKOGRAPHICAL DISTRIBUTION. 113 



the west. Southward of tliese points the temperature of the surface- 

 water continues singularly equable, for the August isotherm of 70° F. 

 is not reached till Cadiz, and that of 77° F. not till N. lat. 20°, far to 

 the south of the Tropic. And the February surface temperatures are 

 equally striking. Water from 40°-49° F. tempers the IS'orwegian 

 coasts as far north as the Arctic Circle, and encloses the whole of the 

 British Isles and the French coast as far south as La Rochelle. 

 Here begins the isotherm of 50° F., coming down from a point off 

 North-West Ireland ; water at 60° F. is not reached on the Portuguese 

 coast till Lisbon, and the isotherm of 68° F. is attained near the 

 latitude of the Cape Yerde, well to the south of lat. 20° N. 



These singularly equable conditions of surface temperature seem to 

 explain many of the prominent features of the distribution of the 

 shallow-water and shore MoUusca of Western Europe. We can 

 understand, on the one hand, how it is that Finmark and the 

 Mourmane coast have a rich littoral fauna,' that warm-water genera 

 such as Pinna and Ileretrix, Ovula and Truncatella, PhasianeUa, 

 Iriforis, Omiebra, Maliotis, and Lotorium reach our own coasts. 

 And we can also understand how northern species have penetrated 

 southward ; how, for instance, Bnccmtim undatiim, L., and Neptunea 

 antiqua, L., reach South-Westerri France, how Littorina littorea, L., 

 reaches the Straits of Gibraltar, L. obtusafa, L., the Western 

 Mediterranean, and Purpura lapillus, L., Algarve and even Mogador. 

 li. T. Lowe remarks^ that of a collection of marine Mollusca picked 

 up on the shore at Mogador, close upon three-fifths are found 

 commonly in Britain. II. McAndrew, dredging in 35-40 fathoms off 

 Mogador, obtained 22 species of shells, 16 of which were British; 

 and of 125 species obtained by him at Madeira, 58 are common to our 

 own shores.^ 



I should like to see the distribution of the marine Mollusca of 

 Western Europe, both in its northern and southern extension, and in 

 range of depth, worked out with the same precision and accuracy 

 as has been done in the case of the Mollusca of the eastern coast of 

 North America. At present there is plenty of enthusiasm, but little 

 organization, plenty of statistics, but no centralized store-room for 

 their preservation. Britain, in virtue of its central position, looking 

 as it does both north and south, and possessing an enormous stretch 

 of coast-land, should take the lead, and I can tliink of no body better 

 fitted to undertake the task of collecting material, sifting evidence, 

 formulating tables of statistics, and keeping them up to date, and 

 generally of acting as a depository of facts and an authoritative court 

 of reference, before which all questions bearing on the subject ought 

 to be brought, than the Society which I am now addressing. The 

 task would be serious ; it ought not to be beyond our powers. 



The British marine molluscan fauna — leaving out of consideration 

 such abyssal species as may be reckoned in the list — is clearly made 



' S. Herzenstein, Congres intern. Zool., vol. ii, pt. ii, pp. 127-47. 

 - Journ. Linn. See, Zool., vol. v, pp. 169-204, 1861. 

 =* Ann. Mag. Nat. Hist., ser. ii, vol. x, pp. 100-8, 18-52. 



