1910.] NATURAL SCIENCES OF PHILADELPHIA. 39 



and that consequently the result of unfavorable conditions, whether 

 applied to the individual or to a local colony, will be within certain 

 limits very similar. 



It is to be noticed that the axial index of the major and minor 

 forms is substantially the same. That is to say, that while there is 

 a well marked division into two races, the variation is mainly one of 

 size and not of proportion. 



3. The Major Races. 



The average of the several "major" series may be compared as 

 follows : 



Alt. Diam. Index. 



1. Cincinnati series 18.78 28.35 .66 



2. Upper Peninsular series 17.59 26.79 .65 



3. Lower Peninsular series 19.44 29.26 .677 



4. General Michigan series 19.06 29.31 .67 



5. Total Michigan series (2 and 3) 18.52 28.03 .665 



Taking the average Cincinnati shell as a basis for comparison, we 

 find that the Upper Peninsular shell is smaller both in height and 

 width and also more depressed; while the Lower Peninsular shell is 

 both higher and wider and proportionately more elevated. This is 

 also true of both series 4 and 5, the latter, however, being very close 

 to the Cincinnati type in every particular. 



As it is a matter of common knowledge that the southern albolabris 

 are usually larger than those from the Northern States, the larger 

 size of the average Lower Peninsular shell seems peculiar. But this 

 is probably owing to the fact that the series is not an "unprejudiced" 

 one, but, as already explained, contains a larger proportion of selected 

 specimens than would occur in a natural series. 



The inclusion of a certain proportion of Upper Peninsular shells 

 in the general Michigan series serves to reduce the average size, and 

 in series 5 a still greater addition of the smaller northern form brings 

 the average down nearly to that of Cincinnati. It seems probable, 

 therefore, that a large and unprejudiced series of southern Michigan 

 shells would show an average not to exceed and quite likely somewhat 

 smaller than the Cincinnati type. 



While it is true that the results obtained from the Lower Peninsular 

 series are perhaps subject to criticism as to size, there does not seem 

 any ground to question the results obtained by a comparison of the 

 axial indices, which show that the Upper Peninsular shell is somewhat 

 more depressed, while the Lower Peninsular type is considerably more 

 elevated than the Cincinnati shell. 



