OP CONCHOLOGY. 231 



PLANORBIS. 



1. Planorbis Hornii, Tryon. — t. 22, f. 16. 



Description. — Shell large, consisting of three convex volu- 

 tions; aperture almost orbicular, not oblique, nor extending 

 above or below the plane of the whorls ; labrum slightly re- 

 flected, thickened within, and approaching so as nearly to 

 connect on the body whorl ; lines of growth fine and close. 

 Color light horn. 



Dimensions. — Height 7 millimetres, diameter 21 mill. 



Habitat. — Fort Simpson, British America. Dr. George H. 

 Horn. 



My Cabinet. 



Remarks. — A very beautiful species, distinguished from all 

 other American forms by its very regular volutions, equally 

 convex above and below, and the rotundity of the aperture. 

 The initial volutions are larger in proportion to the ultimate 

 whorl than in PI. trivolvis, so that they are not much de- 

 pressed below the plane of the surface when viewed either 

 from above or below. There are three volutions. 



2. Planorbis Oregonensis, Tryon. — t. 22, f. 17. 



Description. — Shell very like to PI. trivolvis, but whorls 

 more rapidly increasing in volume; aperture oblique, ear- 

 shaped, very much expanded, extending above and below the 

 plane of the volutions ; texture thin ; stria3 not very close, 

 coarse, and irregular. Color dark horn. 



Dimensions. — Height 4 millimetres, diameter 9 mill, 



Habitat. — Pueblo Valley,* 15 miles south of Camp Alvord, 

 on the boundary between Oregon and Nevada, and 60 miles 

 west of the east boundary of Oregon. W. M. Gabb. 



My Cabinet. Cabinet of Mr. Gabb. 



Observations. — Eesembles much a miniature trivolvis, but 

 may be distinguished by its much more rapidly increasing 

 volutions, and disproportionally large aperture, conjoined with 

 its rugose strige. A large number of specimens are before me, 

 nearly all of the same size as the type figured. 



* "From a thermal spring, water above blood-heat. In the same 

 spring were some aquatic beetles, (Dytiscus,) and extensive growth of 

 Conferva, and swimming on the surface were two species of water birds." 

 — Mr. Gabb's Letter. 



