Correct Position of the Aperture in Planorbis. 



45 



ARTICLE II. — ON THE CORRECT POSITION OF 

 THE APERTURE IN PLANORBIS. 



By Frank C. Baker, B. S. 



In 1 88 1,* Dr. R. E. C. Stearns, in a somewhat exhaustive 

 article, gave as his opinion that the shells of Planorbis were 

 nearly all sinistral and not dextral, as most authors have 

 described them. With a view to establishing this fact beyond 

 a doubt for a number of the species, I set about the examina- 

 tion of the material in the Chicago Academy of Science, with 

 some very interesting results. As suggested by Dr. Stearns 

 in his paper (1. c, p. 96), I examined a large number of 

 Planorbes by breaking away all the whorls save the two or 

 three nuclear whorls. This was comparatively easy for the 

 larger forms, but quite difficult for the smaller species. The 

 result shows what a little time and patience will do in settling 

 a disputed point of this kind. In almost every shell exam- 

 ined, I was able to reduce the adult shell to a form much 

 resembling a Physa with the apex cut off, the apical whorls 

 above and the umbilicus below, the aperture, as in Physa, to 

 the left hand. By reducing all the species to this condition, 

 I was able to see with absolute certainty the position of the 

 aperture. Many of the earlier authors, as Say, Reeve, G. B. 

 Sowerby, Jr., etc., treated the group as sinistral, but most 

 modern authors have considered it dextral, and so figured 

 and described it. That the majority of them were wrong in 

 so describing them, I trust I shall demonstrate in this com- 

 munication. 



Before entering upon a consideration of the different species, 

 it might be well to consider the relation of the shell to the 

 animal, as well as some particular points in the shell itself. 

 The shell is carried perpendicularly when the animal is in 

 motion, thus presenting a right and left side. This fact 



-Are the shells of Planorbis Dextral or Sinistral? Proc. Acad. Nat. Sci., Phil., 

 p. 92, 1S81. 



(Vol. XIX, No. i.) I (Printed August 23, 1S96.) 



