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Or THE UNITED STATES. 161 



which I described in the American edition of Ni- 

 :holson ? s Encyclopaedia, and in the Journal of the 

 Axadeiny. 



Helix albolabris common, as far as Council Bluff. 

 H. thyroidus, on the banks of the Ohio, Mississippi 

 ind Missouri. The Animal is of a pale whitish o 

 yellowish colour, immaculate. 



H. allernata. On the banks of the Ohio, Missis- 

 sippi, and Missouri rivers; this species varies in being 

 somewhat larger, and in having a. rather more eleva- 

 ted spire. The Animal is of a dirty yellowish-orange 

 colour, the foot obtusely terminatedbehind,head and 

 tentacula pale bluish, eyes blackish. Shell 9-10 

 sf an inch in breadth. 



H. hirsuta, common, as far as Council Bluff. 



H. labyrinthica ditto ditto. * 



H. minuta ditto ditto. 



H. perspective occasionally occurs on the banks of 

 the Missouri, and other western streams, and in some 

 parts common. 



Genus Poligyra. 



P. *plicata. Shell convex beneath, depressed 

 above, spire slightly elevated; ivhorls five, compres- 

 sed, crossed hy numerous raised equidistant lines, 

 which form grooves between them; aperture sub- 

 reniform, labrum reflected, regularly arquated, de- 

 scribing two-thirds of a circle, within two-toothed, 

 teeth not separated by a remarkable sinus; labrum 

 with a profound duplicature, which terminates in an 



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