S. Powers — Gastropod Trails in Sandstones. 105 



flakes of mica. Therefore, it is believed that both types 

 of trails were made by the same animal and that the pre- 

 servation of the complete markings is dependent npon the 

 character of the surface over which the animal travelled. 

 The essential means of locomotion was a single foot 

 which pressed deeply into soft mud and lightly into hard, 

 mica-covered sand. The concentric ridges on either side 

 of the central groove in the compound trails are evidently 



Fig. 2. — Some trails of the narrow type. The depressions between the 

 septa-like ridges are very deep. Compare with the central part of the trail 

 shoAvn in fig. 1. Natural size. 



the impression of the shell or body of the animal on either 

 side of the foot and the preservation of these delicate 

 markings was dependent upon a relatively firm surface. 

 In the case of mud these markings were elevated above 

 the central groove and the slightest amount of erosion 

 would remove them while burying the groove. If this is 

 not the correct explanation for the two kinds of trails the 

 fact that certain micaceous surfaces show only the com- 



