388 L. D-. B urling—ProticKhites and Climactichnites. 



and the inferences tliej support follow, nsing the same nota- 

 tion in each case. 



Facts : {a) the side tracks are frequently three-toed ;*^ {Jb) 

 these tritid tracks nsuallj^ toe in ; {c) the side tracks are nsnallj 

 2 or 3 inches apart' ^ though trails up to 5 or 6 inches across 

 have been observed ; {d) the trails are not straight, and a single 

 trail has been observed to reverse its direction entirely so that 

 the animal moved off in a direction parallel to but opposite to 

 that of its previous track, all in a distance of less than three 



Fig. 1. 



Upper Cambrian Trails. 

 ■ Fig. 1. Protichnites loyananits Marsh x 1/6. (After Walcott.) Ausable 

 Chasm, N. Y. U. S. Nat. Mus. 



times the width of its track, the sharpest curve observed hav- 

 ing a radius but little more than half the width of its track ; 

 (e) some of the median grooves are double and very sharply 

 incised, others on the same slab betray no doubling, yet the 

 width of the single groove closely approximates the distance 

 between the double tracks ; (/) the median groove does not 

 swing to the side when the trail makes a turn, even on the 

 sharpest curves the median groove lies midway between the 



