400 FOOTPEINTS OF REPTILIANS. [Ch. XXV 



distances from the next pair. In each parallel row the toes turn the one 

 set to the right, the other to the left. In the European Cheirotherium, 

 before mentioned (p. 337), both the hind and the fore feet have each five 

 toes, and the size of the hind foot is about five times as large as the fore 

 foot. In the American fossil the posterior footprint is not even twice as 

 large as the anterior, and the number of toes is unequal, being five in the 

 hinder and four in the anterior foot. In this, as in the European Cheiro- 

 therium, one toe stands out like a thumb, and these thumb-like toes turn 

 the one set to the right, and the other to the left. The American 

 Clieirotherium was evidently a broader animal, and belonged to a dis- 

 tinct genus from that of the triassic age in Europe.* 



We may assume that the reptile which left these prints on the ancient 

 sands of the coal-measures was an air-breather, because its weight would 

 not have been sufficient under water to have made impressions so deep 

 and distinct. The same conclusion is also borne out by the casts of the 

 cracks above described, for they show that the clay had been exposed to 

 the air and sun, so as to have dried and shrunk. 



The geological position of the sandstone of Greensburg is perfectly 

 clear, being situated in the midst of the Appalachian coal-field, having 

 the main bed of coal, called the Pittsburg seam, above mentioned (p. 

 392), three yards thick, 100 feet above it, and worked in the neighbor- 

 hood, with several other seams of coal at lower levels. The impressions 

 of Lc]ridodendron, Siffillaria, Stigmaria^ and other characteristic car- 

 boniferous plants are found both above and below the level of the reptil- 

 ian footsteps. 



Analogous footprints of a large reptile of still older date were after- 

 wards found (1849) at Pottsville, 70 miles N. E. of Philadelphia, by Mr. 

 Isaac Lea, in a formation of red shales, called No. XL by Prof. H. D. 

 Rogers, in the State Survey of Pennsylvania, and referred by him to the 

 base of the coal, but regarded by some geologists as the uppermost part 

 of the Old Red Sandstone. A thickness of 1700 feet of strata intervenes 

 between the footprints of Greensburg, before described, and these older 

 Pottsville impressions. In the same Red Shale, No. XL, the " debatable 

 ground" between the Carboniferous and Devonian group, Prof. H. D. 

 Rogers announced in 1851 that he had discovered other footprints, re- 

 ferred by him to three species of quadrupeds, all of them five-toed and in 

 double rows, with an opposite symmetry, as if made by right and left 

 feet, while they likewise display the alternation of fore foot and hind foot. 

 One species, the largest of the three, presents a diameter for each foot- 

 print of about two inches, and shows the fore and hind feet to be nearly 

 equal in dimensions. It exhibits a length of stride of about nine inches, 

 and a breadth between the right and left footsteps of nearly four inches. 

 The impressions of the hind feet are but little in the rear of the fore feet. 

 The animal which made them is supposed to have been allied to a Sau- 

 rian, rather than to a Batrachian or Chelonian. With these footmarks 

 were seen shrinkage cracks, such as are caused by the sun's heat in mud, 



* See Lyell's Second Visit, <fec., vol. ii. p. 305. 



