302 



PALEOZOIC TIME. 



of the fore-feet. The print of the fore-feet is something like that of a 

 hand with five stout fingers, the whole four inches broad ; that of the 



Fig. 604. 



Tracks of Sauropus primsevus (x %)■ 



hind-foot is similar, but somewhat smaller, and four-fingered. The 

 Amphibian 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 par- 

 tially obliterated by the tread. The whole surface, including the foot- 

 prints, 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 

 was 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 Rhizopods probably commenced in the 

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

 Baileyi H.. from the St. Louis limestone of Indiana. 



Sponges. — The hornstone of the Subcarboniferous limestones of Illinois and Indiana 

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

 to those of the Corniferous limestone (p. 257). (M. C. White.) Palmacis (Sphenojiote- 

 rium) obtusa M. & W., from the Keokuk beds; P. cuneata M. & W., from the St. Louis 

 limestone. 



2. Radiates. — (a.) Polyps. — Figs. 588, a, Lithostrotion Canadense Castelneau 

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

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



(b.) Echinoderms ; Crinoids. — Fig. 580, Pentremites piriformis Say; Figs. 58], 582, 



