92 PROCEEDINGS OF THE ACADEMY OF [1881. 



OBSERVATIONS ON PLANORBIS. 

 BY ROBERT E. C. STEARNS. 



I. Are the Shells of Planorbis Dextral or Sinistral ? 



Incidental to an investigation into the relations of certain fresh- 

 water snails, upon looking through the books, I find that authori- 

 ties differ on the point, whether the shells of Planorbis are dextral 

 or sinistral. 



While Say,^ Swainsou,^ G. B. Sowerby, Jr.^ and Reeve ^ regard 

 them as sinistral, or reversed, and properly figure the shells, in 

 their works, in a sinistral position, and not " upside down," as in 

 many of the books, Macgillivray^ says ''the shell is dextral, as 

 several observers have proved; not sinistral, as many have 

 alleged;" and Woodward," H. and A. Adams,'' W. G. Binney'^ and 

 others also describe it or refer to it as being dextral. 



Dall remarks in a foot-note to his paper " On the Genus Pom- 

 pholyx and its Allies,"^ " if we consider the shells of this group 

 as dextral, they oflfer the peculiarity of having the genitalia as in 

 most sinistral shells ; Pompholyx presents the same conditions 

 and is certainly dextral." Dr. Philip P. Carpenter, referring to 

 Planorbis^ says, " it lives in a reversed position."^'' 



It will be observed from the above that eminent writers are 

 divided, and that we have substantial authority on both sides of 

 the question. 



My own observations thus far prove the shells to be generally 

 sinistral,^^ but as I have examined but comparativel}' few of the 

 whole number of species, it may be that the shells in some species 

 are dextral, and in other species sinistral. 



1 Say ex Binney, L. and F. W. Shells of N. A,, Part II, p. 103. 



- Treatise on Malacology, p. 337. 



^ Conchological Manual, p. 245. 



* Conchologia Systematica, PI. CXC. 



^ Molluscous Animals of Scotland, p. 114. 



" Manual of MoUusca, second ed., p. 302. 



^ Genera of Recent Mollusca, Vol. II, p. 260. 



'^ Smithsonian Miss. Pub. No. 143, p. 103. 



a Annals of Lyceum of Nat. History of N. Y., Vol. IX, March, 1870. 

 '0 Lectures on Mollusca, S. I. Report, 1860. 



" The figures of Say's larger species in Gould's luvertebrata of Mass., 

 first ed., are most excellent. 



