61 



many as New York; while in the Pleiiroceridd' thi.s comparison 

 is even more marked, the corresponding record being ;^4 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 ecjual. In the Jliigrdphihi 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 I'liioiiiild , r/c/ii'iiccrit/a . and \'ir/jHir/(/ii' are characteristic of 

 the southern region, while the J/i/(jr(ijilii/(i, I'd/rafldd, and 

 Splidriiihi- are characteristic of the northern region. The nnm- 

 ber of species of Sjilmriidd 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 

 l)een made. The writer has obtained records from 69 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. 



CiHiiitien. liirirx. 



Cook 141 Illinois 110 



Fulton 14.5 Wahash 4:5 



La Salle 120 ( Ihio 4.3 



Will 110 Kaskaskia 19 



Mercer 11.'! Kankakee ti" 



Winnebajro T'l Kuck 14 



Mason 9S Kishwaukee 10 



MeHenry SH Mississippi 50 



Menard 02 



Adams 56 



The Cook county list includes S species whi(>h ha\e been 

 inti'oduced from Kurope and are now thriving in the green- 

 houses of t'hicago. 



Too little is as yet known concerning the distribution of 

 Illinois Mollusca to permit any satisfactory generalizations. 

 One of the reasons which i)rompte(l the prepai'ation of the pres- 

 ent catalogue was the hope that its publication might stimulate 

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



