MEDIEVAL OR MESOZOIC TIMES. . 199 



At Fort Pierre, on the Upper Missouri, this group constitutes the 

 hrlls along the Missouri, and extends to the Bad Lands. From 

 Fort Pierre it also extends northward to the Cheyenne and Moreau 

 Rivers, where it dips beneath the Fox Hills group. It also occurs 

 on the Yellowstone. (Meek and Hayden.) , As already observed, it 

 extends from Fort Pierre to the Great Bend, below which, to be- 

 low the mouth of the Niobrara, it rests on the uneven surface of 

 the Niobrara group. This group is met with again on the eastern 

 base of the Rocky Mountains and northward to and beyond the 

 Black Hills. It is seen westward, along the line of the Union 

 Pacific Railroad, on and beyond the Laramie Plains. Where the 

 grayish black carbonaceous shales and marls, and the nearly black 

 arenaceous clays prevail, and no superficial deposits cover them, 

 they give a barren, bleak appearance to the country. (Meek.) 



The thickness of this group on the Upper Missouri is not less 

 than 700 feet. There are a few localities where it is even greater. 

 It was therefore a very long era; so long, indeed, that the ages of 

 human history are as nothing compared with it. During all this 

 time a large portion, and after the middle of the era the greater por- 

 tion of what is now Nebraska was again an extended land surface. 



Life of the Fort Pierre Group Epoch. From the few vegetable 

 remains in the form of petrified and agatized wood that has been 

 preserved, it is evident that the vegetable kingdom was represented 

 mainly by the forms that characterized the preceding era. These, 

 it will be remembered, were mainly cycads, zamias, araucarian con- 

 ifers and tree ferns. 



The animal life of the seas was probably richer than in the pre- 

 ceding era in molluscan forms, and poorer in reptilian life. The 

 Cretaceous, the last period of Mesozoic times, was drawing to a 

 close, and with it its characteristic life. 



Meek has described one echinoderm from this group. He has 

 also described two species of oysters and several varieties. Closely 

 related to the oysters were two Gryphaea and eleven species of 

 Inoceramus. Some of these were of great size and beauty. Ino- 

 ceramus sagensis was nearly six inches long. /. vanuxemi was still 

 larger, being ten inches long and nine inches high. The bivalves 

 seemed to have been specially abundant, as besides the preceding, 

 Meek has described thirty-five species. Thirty species of univalves 

 have also been described. There were many beautiful chambered 

 shells. Two baculites were abundant. Among the eighteen ad- 



