EXPLANATION TO PLATE 28. 



Note. — The shells shown on plates 28-55, except when stated to the contrary, are random 

 samples of the shells used in the quantitative studies. These are considered fairly representative 

 of the shell population of the various groups and of the corresponding parts of the streams. A 

 comparison of these plates with the location of the groups, as shown on plate 61, will enable one 

 to get a concrete idea of the changes of these shells throughout their range. In general the 

 relatively smooth shells are upon the lower parts of the plates. The scale varies, and is indi- 

 cated on each plate. 



POWELL RIVER. 



Group 1. Lot 41. Dryden, Va. This sample is from lot 41 only and does not include 

 lot 45, which inspection shows to be practically of the same character. These are mainly the 

 form powellensis. Natural size. 

 118 



