230 PROCEEDINGS OF THE ACADEMY OF [Mch., 



sculpture is less strong except just behind the lip, where the wrinkles 

 generally are emphatic. 



The lot of smaller specimens (PI. XII, figs. 9-13) consisted of about 

 300 shells. 200 fully adult specimens were measured. The diameter 

 varies between 13 and 17 mm., and gives the curve plotted on p. 229. 

 The major mode is at 15 mm., 43 per cent, of the lot being within .2 

 mm. of that dimension. There is a minor mode at 14 mm.; but the 

 lot is remarkably homogeneous, since over 80 per cent, are between 

 13.8 and 15.2 mm. diameter. 



The altitude bears no constant ratio to the diameter. It is individu- 

 ally variable within wide limits. In the first 50 individuals of 15 mm. 

 diameter (from my schedule of measurements of 200 specimens of all 

 sizes) the following dimensions were found : 



It will be noticed that the mode is at 9 mm., 30% of the whole num- 

 ber having that altitude, while the number of more depressed individ- 

 uals is as nearly as possible equal to the number more elevated. Speci- 

 mens of the same lot, of other diameters, give altitude curves of the 

 same symmetrical form, and need not here be detailed. 



There are 5 albinos in the 200 shells measured, though an uninter- 

 rupted series from albinos to the darkest brown specimens makes a 

 selection difficult. 



Nine shells in 200 show a ver}^ small parietal tooth, one being shown 

 in fig. 12. 



The other lot, of larger shells (PI. 12, figs. 7, 8), contains 2 albinos 

 out of 14 shells. None has a parietal tooth. The measurements 

 follow : 



A small series from Cloudcroft, 8,750 feet, collected by E. O. Wooton, 

 received from Prof. Cockerell, contains also large and small examples, 



