24 PROCEEDINGS OF THE ACADEMY OF [Jan., 



preponderating influence it would have in determining the extent of 

 variation in the shell as a whole. As shown by figs. 5 and 7, however 

 the results, so far as the variation in height and width are concerned 

 would not have been substantially changed. But the size of the 

 average shell would be considerably reduced. 



III. One hundred and fifty-two specimens from Cincinnati, O., 

 collected by the late A. G. Wetherby. This was Prof. Wetherby's 

 duplicate series, and may be fairly considered to be a representative 

 series from that locality. It was used as being the only large series 

 from a single locality available as a basis of comparison with the 

 Michigan specimens. 



IV. Seven specimens of P. albolabris maritima Pils. from Cape 

 May, N.J., viz. : 3 from the collection of Mr. G. H. Clapp; 3 from that 

 of the writer and the type as given by Pilsbry in the original descrip- 

 tion. 



V. Three specimens of P. albolabris minor Sterki from New Phil- 

 adelphia, 0., received from Dr. Sterki. 



VI. Nine specimens of the original lot of P. albolabris traversensis, 

 collected by Leach near Traverse City, Mich. This set is also included 

 in the general Michigan series (I). 



For the purpose of comparing these different series and platting 

 the results, three different measurements have been used: 



1. The height, which is the distance from the apex to the lower 

 base of the lip, measured on a line parallel with the axis. 



2. The greater diameter, measured on a line at right angles to the 

 axis and including the lip. 



3. The ratio of the height to the diameter, obtained by dividing 

 the altitude by the width, the resulting percentage being the axial 

 index of the shell. 5 This method in the case of albolabris gives a 

 reasonable basis of comparison as to the proportionate height of the 

 shell, although it is obvious that it is neither accurate nor satis- 

 factory as a basis for a comparison of shape, as two shells may 

 have the same axial index and yet one be trochiform and the other 

 planorboid. Owing to the comparativeh' low spire and large body 

 whorl of albolabris, no satisfactory method of determining the com- 

 parative elevation of the spire and angular divergence of its sides 

 has been suggested, and attempts in that direction were abandoned 

 as futile. 



«See Baker, Am. Nat., XXXVIII, p. 661 (1904). 



