in the Potsdam Formation. 



283 



These tracks consist of a number of parallel ridges and furrows 

 something like ripple marks, which are arranged betw^een two nar- 

 row continuous parallel ridges, giving to the whole impression a 

 form very like that of a ladder, and as the whole form is usually 

 gently sinuous it looks like a ladder of rope. The surface obtain- 

 ed shews six different trails, (Fig. 1,) the longest of which is about 

 thirteen feet, but they are all of the same breadth, and they may all 



have been impressed by one 

 and the same animal. The 

 breadth of the trails is about 

 six inches and three-quarters to 

 the outer sides of them. 



The transverse ridges and 

 furrows are sometimes straight 

 (Fig. 2,) and sometimes curved 

 (Figs. 3-4-5.) When straight 

 and regular they measure about 

 an inch and three-quarters 

 from the middle of one furrow 

 to that of the next. The height 

 of the ridge is usually from one 

 and a half to two lines, and 

 from the highest part the dis- 

 tance to the middle of the 

 furrows is about an inch and 

 a quarter on one side and half 

 an inch on the other, thus 

 giving to the ridge a sharper 

 slope on the shorter side. The 

 i tops of the ridges, and the 

 || bottoms of the furrows are 

 II somewhat rounded. 



ThouQ-h the transverse ridges 

 are occasionally straight (Fig. 

 Fig.3,0ne.fifthnat.size. 2) they are in general either 



slightly or considerably curved (Figs. 3-4-5), and when so, 

 the chord of the curve is seldom quite at right angles to the 

 direction of the parallel side ridges, one end of the chord 

 in the greatest obliquity observed being as much as two inches and 

 a half in advance of the other (Fig. 3). The height of the curve 



