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of reptilian creatures yet found in the palseozoic rocks of America. 

 The position of the footprints discovered by Dr. King is 1,000 or 

 1,500 feet above the base of the conglomerate, or first member 

 of the coal measures, while these described by Prof. Rogers 

 extend, as has just been stated, some 2,000 feet below that geo- 

 logical horizon. 



Longitudinal markings, looking like trails, probably of mol- 

 lusca, are often found in these rocks, and were exhibited by Prof. 

 Rogers. The most common of these impressions is about half 

 an inch in breadth, and consists of three separate lines of corru- 

 gations, the central band having the corrugations exceedingly 

 minute. Towards one margin of the trail there is invariably a 

 narrow, sharp groove, two or three inches in length, which runs 

 out by a gentle curve towards the edge of the trail, and another 

 commencing within the margin, and terminating likewise by a 

 slight inflection in advance of the first. These, it was suggested 

 as possible, may have originated from the edge or lip of the shell 

 of the crawling mollusk, grooving the softened mud. 



After describing, in a general way, the reptilian footprints in 

 the carboniferous red sandstone of Pennsylvania, assigning the 

 positions which they occupy in the formation, Prof. H. D. Rogers 

 proceeded to offer some reflections, showing the bearings of our 

 present knowledge of the footmarks in the ancient strata gene- 

 rally, on certain cardinal doctrines of geology, especially on the 

 theory of a progressive development in the extinct inhabitants of 

 the earth. He called attention to the fact, that associated with 

 the earlier bird tracks, there are none ascribable to quadrupeds, 

 or any mammalian creatures, while in company with those of the 

 earlier reptiles, occur none attributable to birds. The first birds 

 seem to have appeared about the close of the Triassic or dawn 

 of the great Oolitic period of the middle secondary ages, and no 

 mammalian animals have left either their prints or their skeletons 

 in strata of a date so old ; while of neither bird nor mammal is 

 there print or vestige of any kind in the still more ancient de- 

 posits of the carboniferous and yet earlier rocks, in which the 

 tracks and bones of reptiles and fishes, and the trails and shells 

 of mollusks are of frequent occurrence. Such successive disap- 

 pearance of the traces of the higher forms of life with the advance 



