Plate XL 
Fig. 1. — A large pool in the North Fork of Butcher’s Run, Iowa City, Iowa. 
From this pool were taken .specimens representing every form recorded 
from pools. Photograph taken April 17, 1915, by the author. 
Fig. 2. — One of the characteristic rock shelf rapids of the North Fork of 
Butcher’s Run, almost dry. Photograph taken April 24, 1915, by the 
author. 
