^°1893y''] PROCEEDINGS OF THE NATIONAL MUSEUM. 595 



Tlie Unionida' of the Atlantic sl()[)e are far less vigorous and aggres- 

 sive, and evidently are not fitted to take i)ossession of wide and diver- 

 sified areas. If they o(;cupied any considerable part of the great 

 British American plain before the drift period, it is not at all improb- 

 able that they were well-nigh exterminated by the onward movement 

 of the great cap of ice, which relentlessly ground its Avay from north 

 to south over the face of the country. At the close of the ice age, 

 when this great glacial sheet began to melt away at its southern bor- 

 der, the wajter of this great region, Avhich sloi)ed to the northward and 

 eastward, dammed up by the great ice wall in that direction, was 

 forced over into the ]\rississipi)i through vaiions outlets, and the 

 Unionids of the latter territory, linding an easy entrance into a region 

 almost or quite destitute of other forms, rapidly worked in and became 

 the dominant fauna when the great wall had melted away and the 

 streams resumed their normal courses. 



The absence of the Atlantic species to day throughout a large [)art 

 of the upi)er St. Lawrence region may ])erhai)s be accounted for by 

 supposing that they have never been able to cope with and dispossess 

 their more persitent relatives from the Mississippi Valley, though the 

 evidence afforded by the fossils described in this paper would go to 

 show that, to a certain extent, some of them, at least, had retreated. 



Mr. Ball has called my attention to the important bearing which 

 these fossils may have (if the geological facts stated b(^ fidly confirmed 

 by further exploration) upon the theory of a mild interglacial period, 

 preceded and followed by an advance of the ice. If the ice receded to 

 the vicinity of Toronto, allowing these Mississi])pi species to attain to 

 that region, the fact that they did not establish themselves there 

 would be easily accounted for by the subsequent advance of the ice 

 and the destruction of the colony. The fimxl melting and disa])pear- 

 ance of the ice cap, being complicated by changes in the direction of 

 the drainage, might not afford a second opportunity for the immigra- 

 tion of the si)ecics in <juestion. 



