Pl'JMORDIAL PERIOD. 49 



VESTIGIA. 

 Plate I, fig. C a aud i. 



From the same sti-ata that contain Crusiana Linnarrsoni and G. rustica, 

 the collections contain some thin pieces of siliceous shale marked by a 

 number of series of minute tracks that were probably made by some small 

 Crustacean or other Articulate. They consist of double rows of slight 

 transverse depressions upon the smooth surface of the shale, with a plain 

 space between each series of the double row, about half as wide as the 

 width of a series, but in some cases the two series of tracks constituting 

 the double row nearly meet in the center. Each separate minute track 

 or depression arches slightly, and, although very narrow, they are each 

 nearly or quite as wide as the spaces between them are. In some portions, 

 each separate depression appears as if it had been twice or tluice impressed 

 with minute organs of locomotion of similar size and shape. The width 

 of the double row is about three millimeters, and the transverse length of 

 each separate impression or track is hardly more than one millimeter. 

 There are about twelve unpressions, or separate tracks, in the length of a 

 centimeter. 



The tracks pursue a slightly tortuous course; the longest series shown 

 by the specimens in the collection having a continuous length of about six 

 centimeters, but all of them are broken at both ends. No remains have 

 been found associated with them that might indicate the characteristics of 

 the animal that made fliem; but the naiTOwness and uniformity of the series, 

 together with the fact that each separate minute track appears to have been 

 repeatedly impressed by a series of similar organs of locomotion, seems to 

 suggest vermiform characteristics, but it does not necessarily follow that 

 the animal was a true worm. 



The collections contain no other traces of animal life from these 

 shales, and the formation has been referred to the Primordial period mainly 

 upon stratigraphical grounds. 



Position and locality. — Tonto shale, Primordial period; Grand Canon of 

 the Colorado River, Mohave County, Arizona. 

 4p 



