138 



ELEVENTH REPORT. 



27. Shell elongated peristome reflected Pomatiopsis. 



vSli^ll cbftical, peristome not reflected 28 



5^-#^«.^^ 



P. lapidaria (Say) X3. 



($> 



28. Shell, iimbilicate; outer lip of aperture 



thin Amnicola. 



A. limosa (Say)>;2. 



■Shell with almost no umbilicus, outer lip of aperture 

 thickened. Paludestrina . 



p. nickliniana (Lea) x2. 



29. Land snails, shell thin, translucent, fragile, tentacles ^-^^l^f'^'^Nr 



invertible, with eyes at their tips 30 (^vX^ V\s^ 



Aquatic snails, shell more opaque and heavier, tenta- V,,^:^^^^— ^ 

 cles not invertible, with eyes at bases. .Lymnaea. l. paiustris (Muii.) xi. 



30. Shell distinctlv ribbed Acanthi mil a. 



Shell not ri])bed ....>.. 31 ( 



A. harpa (Say) X6. 



31... Aperture usually more than half as high as entire shell, 



which is three whorled Siiccinea. iVv^ 



S. oralis (Say) XI. 



Aperture about one-third as high as entire shell, 

 ' . which has 5 to 6 whorls Cochlicopa. 



C. liibiica (Mull ) X3j. 



32. Aperture with outer lip regularly rounded. . . . 33 

 Aperture with''> outer lip more or less contracted 

 .-;.?at]«)'^^eithe ' ihiddle, usually strongly indented vvc-o^ 



at -this, giace Vertigo. 



^., - _ V. ovata (Say) Xll. 



33. Aperture edentate 34 



Aperture dentate 38 



34. Last two whorls with prominent thin ribs, first two without ribs. . . 30 

 Shell not more than distinctlv striate 35 



35. Shell mimite, altitude not over 2 mm. . . .Sphj/radium. 

 Shell larger, altitude not less than 4 mm. ... 36 



S. edentulum (Drap ) X11.6. 



36. Aperture almost tAvice as high as wide, peristome thickened. ... 31 



Aperture either almost circular or wider than high, peristome 



reflected 37 



