SUBCARBONIFEROUS PERIOD. 



311 



fore-feet is something like that of a hand with five stout fingers, 

 the whole four inches broad ; that of the hind-foot is similar, but 

 somewhat smaller, and four-fingered. The reptile was therefore 

 large; this is also evident from the length of the stride, which 

 was thirteen inches, and the breadth between the outer edges of 

 the footprints, eight inches. There is also a distinct impression of 

 a tail an inch or more wide. The slab is crossed by a few distant 

 ripple-marks (eight or nine inches apart), which are partially obli- 

 terated by the tread. The whole surface, including the footprints, 

 is covered throughout with rain-drop impressions. 



We thus learn that there existed in the region about Pottsville, 

 at that time, a mud-flat on the border of a body of water ; that the 

 flat had been swept by wavelets leaving ripple-marks ; that the 

 ripples were still fresh when a large amphibian walked across the 

 place ; that a brief shower of rain followed, dotting with its drops 

 the half-dried mud ; that the waters again flowed over the flat, 

 making new deposits of detritus, and so buried the records. 



Characteristic Species. 



1. Protozoans. — Although the class of RMzopods probably commenced 

 in the lowest Silurian, the earliest described species from an American rock is 

 the Rotalia Baileyi Hall, from the Carboniferous limestone of Indiana. 



Sponges. — The hornstone of the Subcarboniferous limestones of Illinois 

 abounds in microscopic spicula of sponges, along with a few Desmids similar in 

 general to those of the Corniferous limestone (p. 271). (M. C. White.) 



2. Radiates — («.) Polyps. — Figs. 521, 522, Lithostrotion Ganadense Castel- 



Fig. 522. 



Figs. 521, 522, Lithostrotion Canadense. 



neau (L. mamillare of some authors, — among whom Milne Edwards, after thus 

 naming it, makes a correction in a note), from the St. Louis limestone. 



