406 Annals of the Carnegir Museum. 



propiiu/uiis, and the latter species is strictly confined to the lake drain- 

 age ; this is an important fact in so far as it demonstrates that no pas- 

 sive transport has played a part in the dispersal of these species, for, if 

 it had been possible for C. ohscums to become colonized by transport 

 (through water-fowl, etc.), in the Erie drainage, the same agents must 

 have worked also in the opposite way, and should have been able to 

 transport C. propiiiqiius into the Ohio drainage. But this is not the 

 case. Our material from the whole immense area occupied by the 

 Ohio drainage contains not a single individual that shows the slightest 

 approach to C. propinqmis, although hundreds of specimens from vari- 

 ous parts have been examined. 



The peculiar presence of C. obscurus in the Potomac drainage at 

 Hyndman is possibly due to similar causes and has a similar bearing ; 

 but since the solution of this tjuestion is to be sought for apparently 

 in parts outside of our territory, we shall here refrain from discuss- 

 ing it. 



It is hoped that the writer may be able to continue and enlarge the 

 above studies, so that it will be possible, in a future memoir, to dis- 

 cuss the whole state of Pennsylvania. If this work is carried out, the 

 distribution of the different species chiefly with regard to their post- 

 glacial immigration into their present range will be elucidated, and, 

 it is needless to say, very interesting results will be obtained, as is 

 already indicated by the above short and preliminary sketch. 



