180 



ANNALS NEW YORE ACADEMY OF SCIENCES 



rarely less. The growth lines are strongly turned back over the fold 

 and sinus. 



Maximum width ; 35mm. 



Minimum width 15 



Maximum length 21 



Minimum length 9 



Highest shell index (adult) 2 . 71 



Lowest shell index (adult) 97 



Average shell index (adult) (about) . . 1.65 



Highest shell index (neanic stage) 4.37 



Lowest shell index (neanic stage) 1.35 



Average shell index (neanic stage) (about) . . 2.50 



The adult curve is fairly short and regular. It corresponds closely 

 with the adult curve of alpenense, though the average index is a little 

 lower. The neanic curve is very broad and irregular. It corresponds 

 rather closely with the neanic curve of alpenense. The average neanic 

 index, however, is a little lower. Two hundred and seventy-six indi- 

 viduals were measured. 



Table 111.— NumTier of individuals and percentages of total number of 

 individuals of various indices 



Index 



-1.00 



1.01-1.25 



1.26-1.50 112 



1.51-1.75 



1.76-2.00 



2.01-2.25 



2.26-2.50 



2.51-2.75 



2.76-3.00 



3.01-3.25 



3.26-3.50 



3.51-3.75 



3.76-4.00 



4.01-4.25 



4.26-4.50 



Total 276 





Adult 



A 





Neanic 



s'umber 



Percentage 



Number 



Percentage 



2 



.7 











36 



13.0 











112 



40.5 



2 



.7 



75 



27.1 



6 



2.1 



40 



14.4 



37 



13.4 



6 



2.1 



72 



26.0 



3 



1.0 



52 



18.8 



2 



.7 



60 



21.5 











31 



11.2 











8 



2.8 











4 



1.4 



























2 



.7 











1 



.35 











1 



.35 



99.5 



276 



99.3 



This mutation is evidently an offshoot from the alpenense stem. The 

 line of development has been more in the nature of a deepening of the 

 sinus and strengthening of the growth lines rather than in reducing the 

 shell index. The shell index is a little lower than that of alpenense in 



