PLATE XXXVIII. 



Casts op trails and markings (p. 63). 



Fig. 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 strise. 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 a is probably the cast of the path 

 of a triiobite walking on the soft bottom. (Collection United States National Museum.) 



ISO 



