Fossil Trades from the Potsdam Sandstone. 279 



posterior points, or by the lateral edges on some kind of a caudal 

 shield. This appears in pi. 1, toward the left above, and in un- 

 figured example?. The most common ones appear as if formed by 

 flexible bristles dragged over the track. They are sometimes 

 wavy, as though the appendages swayed from side to side. 



The third kind of markings is best seen near the sides of the 

 curves, and then only in rare instances. These cross the trans- 

 verse ridges more nearly at right angles and appear to have been 

 formed by bristles of some kind attached to whatever formed the 

 ridges. 



As to the dimensions of the tracks, it may be said that they 

 vary greatly, though all may be grouped in three sizes. The 

 largest are 4-4|^ inches wide,. with a distance of li-lj inches be- 

 tween the transverse ridges, when of the normal form. When 

 there is a curve, the ridges may be much closer towards the inner 

 side of the curve and more distant on the outer side. This size 

 includes the most conspicuous tracks on pi. 1, where one presents a 

 curved course over five feet in length ; also upon pi. 2 and pi. 3, 

 1 below, and 2. The three kinds of finer markings are found only 

 in tracks of this size. The next size is 2-2|- inches across, about h; 

 inch between the ridges, and the smallest size is from 1|- to If 

 inches in width. All of both these sizes show the cross ridges 

 with the Y more or less clearly marked, intercalations, and the 

 usual convex surfaces, with perhaps one exception, which is seen 

 at the bottom of pi. 1. In this one the ridges are flattened with 

 a slight dip forward. This may be formed by a distinct species ; 

 while the others may be conceived as formed by different stages 

 of the same species. 



These tracks closely resemble some described by Sir W. E. 

 Logan, from Perth, Canada, and named by him, Climaciichniies 

 Wilsoni {vid. Greological Survey of Canada, 1863, p. 107). A 

 figure presenting more of the details, is found in Dana's Manual, 

 p. 176. They have fbeen ascribed by different geologists to Mol- 

 luscs, Worms, and Trilobites. These under consideration differ 

 from the Canada tracks, however, in lacking the marginal ridges 

 in all cases, except one very equivocal one. So far as we are 

 aware, they are also without the finer longitudinal markings. 



