ON MOLLUSCA OF THE NORTH-EAST ATLANTIC, ETC. 157 



Concluding Observations. 



The acephalous or bivalve Mollusca possess generally a capacity to exist 

 through a greater bathymetrical range than univalves, several species of the 

 former being to be found in all the zones of deptii from the margin of the 

 sea to a hundred or more fathoms, and it is these same species which are most 

 widely distributed geographically, as might indeed be reasonably inferred, it 

 being evident that the depths of the ocean can be comparatively but slightly 

 affected by changes of temperature and of climate, and that, consequently, a 

 species removed to a distance northward or southward from its most congenial 

 habitat, would encounter less change in climatal conditions by seeking a 

 greater depth. 



Those species which inhabit a great vertical range, such as Saxicava arctica, 

 Venus striatula, Venus ovata, Lucina borealis, &c., have generally their max- 

 imum of development and attain their greatest dimensions in shallow water ; 

 and I call the attention of geologists to this fact as it may occasionally be of 

 service in determining the depth at which strata have been deposited. An- 

 other importantpoint, deserving attention on account of its bearing on geology, 

 is the modifications of growth, incident to all the ijidividuals taken from a great 

 depth, as compared luith individuals of the same species taken from a moderate 

 depth. Some of these vary in different species, but the general characteristics 

 of deep-water specimens are deficiency of colour and of solidity, and small- 

 ness of size. 



Northern species generally diminish greatly in size as they approach 

 southern latitudes ; but the converse of the rule cannot be so generally applied 

 to southern species, for while some of these are smaller, others increase in 

 dimensions as they approach the northern limit of their range. As examples 

 of the latter, 1 may mention Ringicula auriculata and Mactra rugosa, which 

 attain their maximum size in Vigo Bay, Haliotis tiibercidata in Guernsey, and 

 Tellina balaustina in the West of Ireland and the Hebrides. 



To give an idea of the comparatively small number of species existing in 

 high northern latitudes, I may mention that I obtained 50 per cent, more of 

 species in the Canary Islands than in the northern provinces of Norway, 

 although I bestowed at least thrice the amount of time and labour in dredging 

 the latter, under more favourable circumstances, and through a greater range 

 of latitude. 



The correct division of the marine Mollusca into provinces, or as they are 

 called " Faunas," is a subject deserving consideration, as it may be of assist- 

 ance to us in our endeavours to become acquainted with the laws regulating 

 the distribution of species. 



The Arctic and Tropical faunas are tolerably well defined by the zones 

 after which they are named, except that the former, on the European side of 

 the Atlantic, recedes a few degrees within the Arctic Circle, in consequence 

 of the current which sets northward along the coast of Norway. It is the 

 division of the temjierate zone into the Boreal, Celtic, and Lusitanian or 

 Mediterranean provinces, which offers some difficulty, and I take the liberty 

 of submitting the fellowing suggestions with reference to it. 



Two sets of Mollusca of very different type advance from the sub-arctic 

 and sub-tropical regions towards each other. In the course of their progress 

 each loses by the way many of its most characteristic members, which one 

 after another become extinct, so that when they reach their point of contact, 

 the species are comparatively few in number, and not the most characteristic 

 of their northern or southern origin. In order to remedy this state of things 

 and to accomplish an equable distribution of Mollusca throughout the tem- 

 perate zone, it is necessary that there should exist an intermediate fauna, 

 pervading more or less the ground occupied by both the others, and having 



