touch a plane on which the base of the shell may rest ; the 

 aperture also is more transverse. 



KErERENCE TO THE PLATE. 



Fig. L Two views ; natural size, 



p:. 54. 



PLANORBIS TKIVOLVIS. 



DESCRIPTION. 



^/jc// yellowish white, brownish or chestnut color, sub* 

 carinate above and beneath, jDarticularly in the young 

 shell ; ivhirls four, striate across with five, raised, equi- 

 distant, acute lines, forming grooves between them ; spire 

 concave, rather deeply impressed ; aperture large, embra- 

 cing a considerable portion of the penultimate volution; 

 ■imthin bluish white ; labrum a little thickened on the in- 

 ner submargin, and more acutely and rather prominently 

 arquatcd above ; its basal arquation extending below the 

 level of the base ; animal dark ferruginous or dusky, with 

 very numerous, confluent, pale-yellowish points, which 

 also extend upon the tentacula. 



# 



SYNONYMS. 



P. TRivoLvis, S., <Rmer. Ed. of Nic/ioIson''s Encyc.^ Jlr- 



tide Conchologi/i pi. 2, f. 2. 

 C^- :ifXA, TRiuM onniLM, Lister, Conch, pi. 140, f. 46. 

 PjiTi viiu, Gazophijl, pi. 106. f. 17. 



PI. 54. 



