206 Fresh-Water Shells. 



This common shell is well marked by its external simplicity. 

 At the same time, the complicated armature of the aperture, so 

 unique in this family, Avould seem to entitle it to be arranged as a 

 sub-genus. It differs from the preceding in having the umbilicus 

 on tbe left instead of the right side, being its natural place. Mr. 

 Haldeman proposes to make this species the type of a sub-genus, 

 which he calls Planorhula. 



Planorbis IIirsutus. — (Gould.) 



#^ 0l!\ Shell small, somewhat transparent, of a 

 W ®? brownish yellow-color ; both sides concave, 

 the left rather more than the right, but the concavity is there more 

 limited by the presence of a sub-angular ridge on the outer whorl; 

 whorls three, the outer one rapidly increasing ; surface exhibiting 

 traces of revolving lines when denuded, but usually covered with 

 a dark pigment or epidermis, bristling with rigid hairs, which are 

 arranged in close revolving hues ; lines of growth very faint ; 

 aperture sub-oval, oblique, its diameter from side to side shorter 

 than in the opposite direction ; its plane very oblique. Long- 

 diameter i inch, short diameter y'j inch. 



Animal has the head slate-colored above, with a darker line 

 along each tentaculum, not originating from the eyes ; foot 

 chestnut-colored. 



This Planorbis, though in many respects it resembles in shape 

 P. deflectus, is readily distinguished from all other American 

 species by the re\'olving hairy lines. It is the analogue of the 

 European P. albus, from which it is difficult to designate any very 

 characteristic difference. It is, however, "a thinner shell, the last 

 whorl increasing more rapidly; and it maintains its yellowish 

 horn- color, whereas P. albus, assumes a spermacetti or still whiter 

 appearance. The lines, too, disappear more entirely when the 

 epidermis is gone. 



Planorbis Deflectls. — (Say.) 



Shell small, distorted,, compressed, of a light 

 greenish-yellow color, something like diily 

 bleached wax ; right side in general convex, but 

 with the centre slightly indented, suture dis- 

 tinct; left or under side concave, forming an 

 expanded umbilicus, exhibiting about one half of each volution 

 whorls four or five, very much compressed, and reduced to a 



