17 



must by that time have become established along its south- 

 ern shore and in the streams tributary to it in what was then 

 the southern peninsula. Otherwise, there seems to be no rea- 

 son why they should not have established themselves along 

 its northern margin, and from thence spead into the lower 

 waters of the rivers on both sides of the northern part of the 

 state, as they seem to have done in the southern streams. 



With the final disappearance of this channel and the 

 peopling of the streams, which now occupy its bed, the north- 

 ern advance of the greater part of the southern Unionidce 

 seems to have ceased. 



Why all of these species have not continued to spread to 

 the north up the numerous rivers, which now drain the north- 

 ern part of the state, as a few seem to have done, is unknown. 



I have no theory to suggest, except that it may be owing 

 to the same unfavorable conditions of local environment, 

 which have apparently prevented many of them from extend- 

 ing into the head waters of the streams of the southern part 

 of the state, from the margins of the ancient glacial lakes 

 where, if the theories here advanced are true, was originally 

 their home, and from which they have spread so generally 

 into the lower waters of the streams on either side. The fact 

 that some of them have extended their range northerly along 

 the Lake Michigan shore and establish themselves in the 

 lower waters of some of these northern rivers, would seem to 

 substantiate this view. 



In conclusion, I desire to express my obligations to col- 

 lectors of the state and to the custodians of the public mu- 

 seums who, by their cordial co-operation in the work of the 

 census have not only permitted it to be accomplished, but 

 have most materially contributed to whatever measure of suc- 

 cess that has been attained. 



