192 THE MICHIGAN ACADEMY OF SCIENCE. 



smaller brook and creek types of molluscs, suck as tke Pkysas and 

 smaller Limnaeas, persist, as tke rivers are followed down stream, 

 altkougk in fewer numbers. Tkis, kowever, is not tke case in tke Union 

 river. For as tkis river is followed down, tke smaller brook and creek 

 forms give way to tke large Liiiinaea decollata. Tke number of indi- 

 viduals of all species of molluscs is also muck greater in tkis river 

 tkan in tke otkers, botk in tke kead waters and fartker down stream. 

 Tkese diiferences in tke fauna are probably due to tke ditferences in 

 tke rocks over wkick tke rivers flow; for tke rivers seem to differ in no 

 otker way and all empty witkin a few miles of eack otker. Witk tke 

 exeej)tion of tke Union, tke rivers of tkis region flow tkrougk sand- 

 stone and skale, rocks containing practically no lime. Tkese streams are 

 tkerefore, poor in individuals and unable to support tke larger river snails, 

 wkick recjuirc a lai-ge amount of lime in tke construction of tkeir keavy 

 skells. Union river, on tke otker kand, drains, on tke east, a ridge com- 

 posed of melapkyre lying between two beds of sandstone. Melapkyre 

 is an eruptive rock wkick contains a large amount of lime and con- 

 sequently Union river probably contains nmck more lime tkan tke other 

 streams explored. A fact Avkick is borne out by tke larger number of 

 individuals found kere and by tke presence of Liiiinaca decollata in suck 

 abundance. 



Mr. Walker, in a letter, commenting on tke molluscs of tkis region 

 says : "Tke fauna of Ontonagon county is very similar to tkat of Mar- 

 quette county around Huron mountain, but tke list is not so large ow- 

 ing to tke fact tkat Huron mountain kas been more tkorougkly worked. 

 Tke fauna is essentially a nortkern one. Tke countj^ not being far 

 enougk west to be affected by tke upper curve of tke Transition Zone 

 wkick, according to Merriam, touckes tke soutkwestern extremity' of 

 Lake Superior. Just wkere tkis line is to be drawn, kowever, is, I im- 

 agine, a very dubious question." 



I am under special obligations to Mr. Bryant Walker, at wkose ex- 

 pense tkese collections were made, for tke privilege of reporting on tke 

 skells and for tkeir names. My tkanks are also due to Dr. A. C. Lane, 

 State Geologist of Mickigan, wko extended to me tke kospitality of tke 

 camp of tke State Geological Survey party, and to Dr. F. E. Wrigkt 

 and tke otker members of tke party for tkeir courtesy and assistance in 

 tke field. 



I am also indebted to Mr. Ckas. C. Adams, Curator of tke Museum at 

 the University of Mickigan, for assistance in tke prosecution of tke work 

 and in tke preparation of tkis paper for publication. 



Tke skells collected are in Mr. Walker's collection at Detroit, and tke 

 reptiles and Ampkibians are in tke University Museum at tke University 

 of Mickigan. 



A. G. RUTHVEN^ 



University Museum, University of Mickigan, 

 Ann Arbor. 



