Marine Zoology, 



It is evident that the capacity of bivalves to enjoy a great bathy- 

 motrical range exceeds considerably that of univalves. This power 

 of enduring many conditions of depth implies the power of adapting 

 themselves to varying circumstances, which cannot be supposed to 

 exist without considerable variation in the features of the individuals 

 of such wide-ranging species. The rules which should guide us in 

 determining the selection of diagnostic characters from the shells of 

 Acephalous mollusks, must consequently be less strict than those 

 which should determine our selection of characters for the majority 

 of Gasteropoda ; and in the determination of fossil species this should 

 constantly be borne in mind. The difference of power to range pre- 

 sented by univalves as compared with bivalves, has a further im- 

 portant bearing on palseontological inquiries, for it would indicate 

 the probability of our not unfrequently finding geological formations 

 connected together by the fossils of one class of mollusca, whilst those 

 of the other are altogether distinct, even in strata proximate in time. 

 It is possible, also, that by a careful determination of the relative 

 proportions of bivalves to univalves in ancient sea beds, all mineral 

 indications of the nature of the sea-bed being at the same time noted, 

 we may get an additional clue to the determination of the depth of 

 the ancient sea in which such animals lived. 



The distribution of the sub-littoral forms of Testacea, is illustrated 

 by the following examples : — 



Certain species are common to the Laminarian, Coralline^ and 

 Deep-sea Coral zones, — as Pinna ingens, Buccinum undatum, &c. 



Certain species are common to Laminarian and Coralline zones, 

 and indifferently inhabit both, — as, Cyprsca europea, Ostrea edulis, 

 Pecten varius, &c. 



Certain species commence their range in the Coralline zone, — as, 

 Rissoa abyssicola, Fusus islandicus, &c. 



How far the nature of the sea-bottom determines the number and 

 diffusion of species.* 



Now, though our evidence certainly goes to show that the range 

 of species in depth and distance from shore is often considerably 

 extended by a continuity, whether vertical or horizontal, of the same 

 kind of ground ; yet assuredly ground alone will not determine 

 the extension of any species ; for otherwise we should have the stone 

 and gravel inhabiting species of the Littoral zone carried in many 

 places into the Laminarian and Coralline zones, and the peculiar 

 inhabitants of the muddy and sandy tracts in the Laminarian zone 

 carried far into the depths of the sea, since in very many places 

 these kinds of sea-bed range without interruption from shallows to 



* In the Tables in the Report, the nature of the sea-bed is expressed by 

 letters representing the several mineral characters of the bottom, whether sand, 

 sandy mud, mud, rock, stones, gravel, muddy gravel, shelly, shell sand, or 

 nulliporc ; the last kind of bottom being that commonly called " coral," in the 

 charts of the European seas. 



VOL. LI. NO. ClI. — OCTOBER 1851. 2 C 



