61 



many as New York; while in the Pleuroceridce this comparison 

 is even more marked, the corresponding record being 34 times 

 as many for New York, 30 times as many for Michigan, and 

 about 11 times as many for Illinois. In other families the ra- 

 tios are more nearly equal. In the Hygrophila the greatest in- 

 crease is toward the west, Michigan and Illinois having the 

 greatest number of species. In a general way we may say that 

 the Unionidce, Pleuroceridce, and \ r irijxiri</<r are characteristic of 

 the southern region, while the Hygrophila, Valvatidce, and 

 Sphceriidce are characteristic of the northern region. The num- 

 ber of species of Sphceriidce found in this state (42) is notewor- 

 thy. 



A study of the published and manuscript lists of the state 

 shows that little is known concerning the geographical distri- 

 bution of our Mollusca. For only ten counties has a careful list 

 been made. The writer has obtained records from G9 counties, 

 the number of species ranging from 1 to 145. The following 

 list shows the number of species known to inhabit 10 counties 

 and 8 rivers. 



Counties. 



Cook 141 



Fulton 145 



La Salle 120 



Will 110 



Mercer 113 



Winnebago 75 



Mason 98 



McHenry 89 



Menard 62 



Adams 56 



Rivers. 



Illinois 110 



Wabash 43 



Ohio 43 



Kaskaskia 19 



Kankakee 67 



Rock 14 



Kishwaukee 10 



Mississippi 56 



The Cook county list includes 8 species which have been 

 introduced from Europe and are now thriving in the green- 

 houses of Chicago. 



Too little is as yet known concerning the distribution of 

 Illinois Mollusca to permit any satisfactory generalizations. 

 One of the reasons which prompted the preparation of the pres- 

 ent catalogue was the hope that its publication might stimulate 

 a desire among conchologists to prepare careful, up-to-date lists 



