Cii. XXY.] AIR-BREATHLXG REPTILES. 509 



the impressions ; whereas, when it afterwards dried up and shrank, it 

 would be too hard to receive such indentations. 



No less than twenty-three footsteps were observed by Dr. King in 

 the same quarry before it was abandoned, the greater part of them so 

 arranged (see fig. 560) on the surface of one stratum as to imply that 

 they were made successively by the same animal. Everywhere there 

 was a double row of tracks, and in each row they occur in pairs, each 

 pair consisting of a hind and fore foot, and each being at nearly equal 

 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 Cheiro- 

 therium, before mentioned (p. 443), 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 foot- 

 print 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 Cheirotherium, 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 Cheirotherium was evidently a 

 broader animal, and belonged to a distinct 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, hav- 

 ing the main bed of coal, called the Pittsburg seam, above mentioned 

 (p. 501), 3 yards thick, 100 feet above it, and worked in the neigh 

 borhood, with several other seams of coal at lower levels. The im 

 pressions of Lepidodendron, Sigillaria, Stigmaria, and other charac- 

 teristic carboniferous plants are found both above and below the level 

 of the reptilian footsteps. 



Analogous footprints of a large reptile of still older date were 

 afterwards found (1849) at Pottsville, 10 miles JNT.E. of Philadelphia, 

 by Mr. Isaac Lea, in a formation of red shales, called jSTo. XI. by 

 Prof. H. D. Eogers, in the State Survey of Pennsylvania, and re- 

 ferred by him to the base of the coal, but regarded by some geolo- 

 gists as the uppermost part of the Old Red Sandstone. A thickness 

 of 1700 feet of strata intervenes between the footprints of Greens- 

 burg, 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 



* See Lyell's Second Visit, &c, vol. ii. p. 305. 



