644 



HISTOEICAL GEOLOGY. 



Keokuk limestone. The Cestracionts (see page 416), with a rough, uneven 

 pavement, were represented by species of Helodus and Oroclus. Some of the 

 sharp-pointed teeth of Hyhodonts are shown in Figs. 1020-1022 (Newberry 

 and Worthen). 



1018. 



1019. 



Teeth of Cestkaoiont Sharks. —Fig. 1018, Cochliodus nobilis ; 1019, C. contortus (x |). Fig. 1018, Meek ; 



1019, Agassiz. 



Fin-spines of Sharks are various in size and form. One, of Ctenacanthus, 

 has a length of a foot; and others, now broken, were probably 6 inches longer ; 

 they indicate fins of large size, and therefore the existence of great Sharks. 



Teeth op Sharks. 



1020. 



1020-1022. 



1021. 



■ Fig. 1020, Carcharopsis Wortheni ; 1021, Cladodus spinosus ; 1022, Orodus mammillaris. 

 Newberry. 



Amphibians Sire "known irom their footprints on a layer of the Mauch 

 Chunk shale near Pottsville, in Pennsylvania, as described by Isaac Lea. 

 A reduced view of the slab is shown in Pig. 1023. There is a succession of 

 six steps, along a surface little over five feet long ; each step is a double one, 

 as the hind-feet trod nearly in the impressions of the fore-feet. The prints 

 were hand-like ; that of the fore-foot five-fingered and four inches broad ; that 

 of the hind-foot somewhat smaller, and four-fingered. That the Amphibian 

 was therefore large, is also evident from the length of the stride, which was 

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



