38 PROCEEDINGS OF THE ACADEMY OF [Jan., 



That is, the average Upper Peninsular shell is smaller by 1.08 mm. 

 in height and 1.29 mm. in width and proportionately more depressed 

 by .22 mm. 



2. The Minor Races. 



The several series of the " minor" form may be tabulated as follows, 

 using the average shell as a basis of comparison: 



Alt. Diam. Index. 



1. maritime! 15.60 22.70 .68 



2. minor Sterki 14.25 21.25 .67 



3. traversensis 13.31 21.31 .63 



4. Upper Peninsular series 15.44 23.31 ' .66 



5. Lower Peninsular series 14.04 22.18 .66 



6. General Michigan series 14.67 22.30 .66 



7. Total Michigan series (4 and 5) 14.74 22.75 .66 



From this table it will be seen — 



1. That all the Michigan shells of the minor form are more depressed 

 than the typical maritima and minor Sterki. This is specially true 

 of the typical traversensis. 



2. That, with the exception of the typical traversensis series, all 

 the Michigan series have the same index. It is to be noted, however, 

 that the traversensis series is included in series 5, 6 and 7, and that, 

 if that series was eliminated, the index of these series would be some- 

 what increased and more closely approximated to that of maritima 

 and minor Sterki. 



3. That the Upper Peninsular series average larger than that of 

 the Lower Peninsula by 1.4 mm. in height and 1.13 mm. in width. 

 This is in marked contrast with the major forms of the two series, 

 in which the Lower Peninsular shell is larger by 1.08 mm. in height 

 and 1.29 mm. in width. This is well shown by comparing the two 

 curves in fig. 15. 



4. On the whole, however, the several series show a remarkable 

 uniformity. It would seem to be reasonably clear that the minor 

 form of albolabris varies in about the same way within certain fairty 

 defined limits. There is reason to believe that in certain locations 

 these peculiarities affect the whole race, and in such cases is worthy 

 of varietal recognition. It is equally true, no doubt, that many of 

 the minor shells are merely depauperate individuals. But whichever 

 is the case, so far as size and proportion are concerned, the amount 

 and range of variation is substantially the same. The explanation 

 of this probably is that the amount of depauperization that the 

 species will sustain and still exist is substantially the same everywhere, 



