i.i.As (hi tlif Ori'</in f /'/. x/ifti/i-f t\inna9. Ill 



from early Pliocene frt-sli\vatrr ancestors. It i>, on the other hand, possible that 

 tin 1 modification i .-<>me marine Littorinid into Limnotrochus, as well perhaps as 

 S\nu.l:> intii Syniolopsis. may have taken place during the conversion of some 

 part of the Tertiary MM into the African lake's ; liut whether these two genera 

 ial modifications confined to the African area or not, future research alone 

 can deride. 



The lakes of the northern hemisphere must now be briefly referred to. Some 

 of them, as those of Norway and the British Isles, were probably submerged 

 lieneath the sea during the middle of the glacial episode. The same is probably 

 true of the Xorth American lakes. This short, temporary submergence, while it 

 must have destroyed the freshwater inhabitants of the lakes, probably introduced 

 into them the marine and Arctic Crustacea, Mysis relicta and Pontoporeia affims, 

 which IK came isolated from the sea on the subsequent re-emergence of the land. 

 The rest of the present inhabitants of these lakes must have been subsequently 

 .-applied l>y th which discharge into them. It is probably owing to the 



< 'Innate which culminated in glacial conditions that the Melanidae are no 

 longer to be found in Northern Europe. The cold and the glaciers would, no 

 doubt, have operated quite as effectually in North America; but in this case the 

 structure of the country has made it possible for the Strepomatidae to return from 

 their southern exile, and once more to occupy their pre-glacial habitat. The great 

 size also of the American lakes might have saved the Melanidse from extinction 

 during tin- first and most severe part of the glacial period; and during the great 

 submergence, when the climate was warmer, they might have sought refuge from 

 the sea-water in the freshly restored river-systems. 



We have, finally, to consider the causes which have led to some of the more 

 marked modifications which characterise freshwater genera. In the first place 

 they are seldom shared by their nearest marine relations: no marine mollusc is 

 known to pass through a glochidium stage ; no marine Polyzoon nor sponge produces 

 a winter bud or statoblast ; no marine Phyllopod an ephippium ; and no Tubula- 

 rian an egg within a horny shell like that of Hydra. A large number of marine 

 molluscs, however, lay their eggs in capsules, and some, such as Cymba, are vivi- 

 parous. 



The winter eggs of sponges and Polyzoa, since they appear in correspondence 

 with a seasonal change, are probably, as Semper has suggested, produced as a pro- 

 tection against cold or drought. If they are adaptations to a freshwater climate 

 they must have appeared subsequently to the isolation of the organism from the sea ; 

 and thus, though now available as a means of distribution, could not have been the 

 means by which the organisms producing them exchanged a marine for a freshwater 

 habitat. The ephippium of Daphnia and the incapsuled embryo of Hydra may also 

 be regarded as modifications induced by the severity of a freshwater climate. The 

 fact that the embryo of Hydra hatches out soon after it is laid in a freshwater tank 



TRASS. BOY. DUB. 8OC., N. g. VOL. 111. R 



