31 



Shell dextral, spire depressed ; whirls few, visible on 

 both sides, furnished internally with teetli ; outer lip 

 simple. 



SUBGENUS PLANORBULA, Halde. 



SE<i4:?IK.>TI.^A AKm«KKA, Say. 



Shell lias four wliirls, umbilicus deep, exliibiting the 

 volutions ; aperture subovate ; oblique ; far within the 

 throat six teeth ; color of shell brownish ; breadth one- 

 fourth of an inch. Abundant. 



SUBFAMILY ANCYLINiE. 



OEx-VlJ.*} A.^CYf.US, Gkokkhoy. 



River-limpet. Etymology, Ancyhis {aglculos,) a 

 small round shield. 



This genus resembles in shape the marine jiatella. 

 The shells are fragile and small. We have a species 

 which is about two-sixteenth of an inch in length, and 

 which I presume to be Ancylus tardus, Say. I found 

 numbers of thorn associated with a valvata-like 

 Pltrygania, clinging to stones in Fox River. 



ORDER PECTINIBRANCHIATA. 



FAMILY VALVATIDvE. 



«e:\i;s vai.vata, Mui.lek. 



Valvai» tricarinata. Say. Shell lias three volutious; 



three revolving, carinate, prominent lines, giving to the 

 whirls a quadrate appearance ; Suture deep ; umbilicus 

 large ; aperture circular ; lip simple, almost surround- 

 ing the aperture ; shell horn colored ; breadth one- 



