THOMSON: ON HELIX (mESODON) CHILHOWEENSIS. 273 



the individual so too the species and the genus, have their birth, 

 their vigour, their decay, and their death, and that nature's 

 moulds themselves are as impressionable as the receiving surface 

 of the photograph — not one external influence but evokes a 

 corresponding modification. 



NOTE ON THE SPECIFIC DISTINCTNESS OF HELIX 

 (MESODON) CHILHOWEENSIS, Lewis. 



By JOHN H. THOMSON, C.M.Z.S. 



In examining some magnificent specimens of Helix {Mesodon) 

 chilhoweensis Lewis, (vide Proceed. Philad. Acad. Nat. Sci. 1875, 

 p. 335) received from and collected by Mrs. Jud^e Geo. Andrews, 

 in the mountains of Tennessee, and comparing them with Helix 

 (Mesodofi) Sayi Binney, of which species it is supposed to be only 

 a larger growth, by Mr. W. G. Binney, I note on the two last 

 whorls, revolving stride, decussating with the ribs of growth, which 

 I am unable to find on any specimen of Helix Sayi even the var. 

 major. I also note on those specimens furnished with "the faint 

 rudimentary tooth near the umbilicus" vide 1. c, that it differs ma- 

 terially from the tooth on the lower part of the aperture in H.Sayi, 

 in fact the tooth in H. Sayi appears more like a fold with a cor- 

 responding constriction on the outside of the shell, forming in fact 

 a slight superficial groove running round on the umbilicus. The 

 description of the species by Dr. Lewis, must be somewhat amended 

 as he says, (Proc. Philad. Acad.) "parietal wall without any indi- 

 cations of a tooth." In about one-half of my specimens I find a 

 large obtuse tooth on the parietal wall, in fact in one specimen I 

 found two teeth, one bifid and further within the shell on the pa- 

 rietal wall, but both very different from the parietal tooth oi Helix 

 Sayi. Diameter of my specimens from 35 to 41 mill. 



Hab. Smoky Mountains, North Carolina, and Cumberland 

 Mountains, Tennessee. 



