MEDI.EVAL OR MESOZOIC TIMES. 189 



this epoch. The eastern border, at least, of the Cretaceous area, 

 was the eastern shore line of the interior sea of the time. 



Vegetable Life of the Niobrara Group Epoch. — The diatoms and 

 desnuds which abounded in some strata in the European chalk, 

 were sparingly represented in the Niobrara Group seas. I have 

 only in a single instance found a few diatoms under the microscope 

 in some chalk obtained below the mouth of the Niobrara River. 

 The specimen was overlaid by a portion of the skeleton of a fish 

 which seems to have protected the silicious matter which had ac- 

 cumulated and which contained the diatoms. 



The peculiar impressions of geologically modern leaves (dicotyle- 

 donous) which characterize the Dakota Group, are wanting in the 

 Niobrara. Different seas now prevailed, and as is evident from the 

 fossil animals, to be noticed hereafter, a warmer climate. Only one 

 leaf impression, to my knowledge, has been obtained from this 

 group in Nebraska. It was found in the Inoceramus bed in Dakota 

 County, by Hon. Jesse Warner, and presented to me for the cabinet 

 of the State University. Owing to the absence of nerve marks, it 

 could not be certainly identified, but its external form was that of a 

 laurus. 



Fossil wood, however, is abundant, both petrified and agatised. 

 Of this material I have made microscopic sections of seventv-nine 

 specimens, which under the microscope showed the structure of the 

 original wood. Of these seventy-nine specimens, forty-seven be- 

 longed to the conifers of araucarian type, and the balance were 

 cycads and zamias. Judging only from these few remains, the di- 

 cotyledonous vegetation that characterized this region in Dakota 

 Group times, had retreated, where to is not certainly known, but 

 probably northward or northeastward. A southern flora, or one 

 that had reached its culmination in Jurassic times, returned again 

 to this region by migration. At the same time a few species from 

 the Dakota Group era lingered among these mediaeval vegetable 

 forms. 



Animal Life of the Niobrara Group Epoch. — The chalk of Europe 

 was largely made up of remains of rhizopods which were so abun- 

 dant that a cubic inch, according to Ehrenberg, contained millions 

 of these low organisms. In our own chalk seas they were probably 

 little less abundant, though not so well preserved. Some specimens 

 of chalk that I obtained below the mouth of the Niobrara, and in 

 Cedar County, afforded them, under the compound microscope, in 



