FOOTPRINTS ON THE SANDS OF TIME 35 
important point is that there are no marks of toes or nails. Their 
edges are not sharply defined, but are rounded off, and there is a 
slight variation in the form and depth of the corresponding im- 
pressions on each side of the furrow. But the reader will see 
from the figure that they do correspond with each other. Thus, 
take the three tracks at the bottom of the lower group on the 
right side of the furrow ; the innermost of this group of three may 
Fig. 1. — Tracks (Proticlmites), probably of a crustacean, from the Potsdam 
Sandstone, North America. (After Owen.) 
easily be identified with the innermost track of the group of three 
on the left side of the furrow. And so with the two groups of 
three belonging to the two upper sets of impressions, each of which 
is enclosed in an oval. 
These very ancient tracks are known to geologists under the 
name Protichnites, and the creature that produced them must have 
made no less than fourteen impressions, seven on the right and 
