LIMNEANA. MOLLUSCA. PLANORBIS. 205 



This shell does not attain the size of the preceding species ; and, 

 when mature, its dilated throat distinguishes it from every other 

 known species ; and the remarkable manner in which it is turned, as 

 it were by violence, so as to look to the left, is a still further distinc- 

 tion. The outer whorl is everywhere of the same breadth ; and the 

 immature shell, before the dilatation of the throat, may be known by 

 the very regular enrolment of the whorls, and the very contracted 

 aperture in consequence of the very unequal diameters. 



Planurbis armigerus. 



Shell flat on the right side, and concave on the left ; whorls 

 four, loith minute revolving lines on the concave side ; throat with 

 five unequal teeth far within the aperture. 



Figure 138. 



State Coll., No. 80. Soc. Cab., No. 1264. 



Planorbis armigerus, Say; Journ. Jlcad. JYat. Sc, ii. 164. 



Shell small, brownish horn-color, or light-chestnut, orbicular ; 

 right side nearly plane, with only a slight central pit, showing four 

 rounded volutions, distinctly separated by the suture ; left side 

 deeply concave, exhibiting all the whorls, which on this side are 

 sub-carinated ; surface shining, faintly marked by the lines of 

 growth, and, on the left side, may be distinctly seen several 

 raised revolving lines on each of the whorls ; aperture slightly 

 inclining to the left, rounded, and very slightly modified by the 

 carina, very oblique ; edge of lip dark-brown ; at some distance 

 within the throat are five white teeth, nearly closing the passage ; 

 a large, prominent, oblique one is situated on the side of the pre- 

 ceding whorl, and may always be seen ; a very small one is by 

 its side ; opposite to them are the three others which are small. 

 Larger diameter -^^ inch, smaller diameter y^ inch. 



Animal very active, of a blue-black or slate-color ; foot long 

 and narrow. The shell is carried inclined at an angle of 45'^. 

 The respiratory groove is very acutely pointed. 



Found abundantly in shady, stagnant pools and ditches, in which 

 an abundance of decaying vegetable matter is immersed. 



This common shell is well marked by its external simplicity. At 

 the same time, the complicated armature of the aperture, so unique in 



