| eg Byes BO. O.@-AICIE TL 
CASTS OF TRAILS AND MARKINGS (p. 63). 
Fic. 1. View of a slab of Tonto (Middle Cambrian) sandstone from the Grand Canyon of the 
Colorado. The principal trail is the large one with coarse, parallel strie. This strongly 
suggests the drifting over the surface of the mud of some medusa-like form with numerous 
trailing tentacles. The strong trail which terminates at wis probably the cast of the path 
of a trilobite walking on the soft bottom. (Collection United States National Museum.) 
180 
