106 



GEOLOGY OF lyA SAIvIvE COUNTY. 



in this count3\ In the Lake Michigan drainag-e around 

 Chicagfo Physa g-yrina attains a very larg-e size, but is 

 different in shape from Physa Hildrethicma. 



GENUS BULINUS, ADANSON. 



Shell frag'ile, slender, polished; whirls six, aper- 

 ture narrow, spire tapering-, color lig-ht yellowish, col- 

 umellar fold slig-ht; leng-th about 18 mill. Loc. stag- 

 nant ponds near Illinois river. 



Liffers from Physa, in being- more slender and 

 more hig-hly polished, and in having- a simple, unfring-ed 

 mantle; appears of a deep black when the animal 

 is within. 



SUBFAMIIvY PLANORBIN^. 



Genus Planorbis; Guettard — One of our most 

 abundant moUusks, is found usually in quiet waters 

 feeding- on confervaceous vegfetation or decayed matter. 

 The g-enus is represented by a number of species in 

 this county. I believe that more species of Planorbis 

 have been made than will stand. Several varieties of 

 P. trivolvis are very g-ood as varieties, and I believe as 

 much entitled to the rank of species as some that are 

 called such. 



Planorbis g-labratus. Say. — Shell has five whirls, 

 rugose in appearance, spire regular; umbilicus large, 

 deeply concave; exhibiting the volutions; color brown- 

 ish; breadth nearly one mch. Loc. Fox River. 



SUBGENUS PLANORBElvIvA, HALDE. 



Planorbis campanulatus, Say. — Shell has four 

 whirls; umbilicus deep, color yellowish horn; aper- 

 ture dialated, campanulated; greatest breadth one-half 

 inch. Abundant. 



