48 THE GEOLOGICAL HISTORY OF PLANTS. 



land and water, and a mild and equable climate, permit- 

 ting the existence of a rich vegetation in high northern 

 latitudes. Of this latter fact a remarkable example is 

 afforded by the beds holding plants of this age in Spitz- 

 bergen and Bear Island, in its vicinity. Here there seem 

 to be two series of plant-bearing strata, one with the 

 vegetation of the Upper Erian, the other with that of 

 the Lower Carboniferous, though both have been united 

 by Heer under his so-called "Ursa Stage," in which he 

 has grouped the characteristic plants of two distinct 

 periods. This has recently been fully established by the 

 researches of Nathorst, though the author had already 

 suggested it as the probable explanation of the strange 

 union of species in the Ursa group of Heer. 



In studying the vegetation of this remarkable period, 

 we must take merely some of the more important forms 

 as examples, since it would be impossible to notice all 

 the species, and some of them may be better treated in 

 the Carboniferous, where they have their headquarters. 

 (Fig. 15.) 



I may first refer to a family which seems to have cul- 

 minated in the Erian age, and ever since to have occupied 

 a less important place. It is that of the curious aquatic 

 plants known as Khizocarps,* and referred to in the last 

 chapter. 



My attention was first directed to these organisms by 

 the late Sir W. E. Logan in 1869. He had obtained from 

 the Upper Erian shale of Kettle Point, Lake Huron, 

 specimens filled with minute circular discs, to which he 

 referred, in his report of 1863, as "microscopic orbicular 

 bodies." Kecognising them to be macrospores, or spore- 

 cases, I introduced them into the report on the Erian 



* Or, as they have recently been named by some botanists, " Hete- 

 rosporous Filiees," though they are certainly not ferns in any ordinary 

 sense of that term. 



