222 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Mar. 24, 



The average breadth of the track from the outermost side of the 

 outer impressions is 5 inches. From the median track to the outer- 

 most of the outer impressions is 3 inches, and to the innermost of the 

 same pair of impressions 1 inch 3 hues. The longitudinal extent of 

 one of the curves which includes five sets of prints is 7 inches. The 

 interrupted impressions of the median track show a slight deviation 

 from the straight line. 



The modifications presented by this series of impressions equally 

 militate against their having been left by a vertebrate animal, but 

 differ so much from those already described as to clearly indicate a 

 distinct genus of many-limbed animal. 



There are four series of impressions on a great extent of ripple- 

 marked sandstones, in one of which the median track has cut through 

 the ripple-marks along an undulating curved line of nearly equal and 

 considerable depth, not showing the alternate rise and fall which is 

 seen in so many of the other sets of impressions. The margins of 

 this median track are rounded off, and it is more rounded at the 

 bottom. The lateral tracks are large, shallow, and faint, as if they 

 had been partially obliterated by the action of water ; yet the prints 

 can be still distinctly traced, indicating a total and regular breadth 

 of 6 inches across the whole of the impressions. 



Along another extent of ripple-marked slab, a narrower median 

 impression cuts through the ripple-risings for an extent of 7 feet in 

 nearly a straight line. Here also the lateral impressions are faintly 

 indicated, their borders being rounded off and as it were expanded, 

 showing a total breadth of 5 inches, across the tracks. As the sand 

 appears to have been of a dense siliceous character, the ripple-marks 

 could have only been ploughed through to the depth shown by these 

 impressions, by a pretty considerable momentum, either of velocity or 

 of weight, occasioned by the moving animal. 



Along a third extent of ripple-marked surface the median im- 

 pressing part of the moving body has left only a narrow and shallow 

 impression upon the summits of the successive sand-waves, the direc- 

 tion of the animal being shown by that in which the sand had been 

 pushed into the intervening valleys of the ripples. 



With these varied and well-marked evidences of the number, form, 

 grouping, and arrangement of the foot-prints impressed upon the 

 Potsdam sandstone, in the more clearly impressed specimens, now 

 submitted, both in the original sandstones and in good plaster-casts, to 

 our inspection, we may readily discern a general correspondence with 

 them, of those comparatively more confused and obscurely marked 

 impressions (PI. XIV. A.), the casts of which were first brought 

 over by Mr. Logan during the preceding year. The foot-print oc- 

 casionally occurring on the inner side of the pairs of prints, may now 

 be recognized as answering to that marked c in PL X., which forms 

 the innermost impression of the regularly recurring group of 3, 

 viz. c, c', c". It is not, as I at first supposed, the result of the fore- 

 leg being applied to the ground a second time, on the inner side of 

 the first step, during a temporary stop in the animal's progress. 



