2 THE PALEONTOLOGY OF MINNESOTA. 



[Geological introduction of plants. 



ferns) and the Lycopodiacece. It is very probable that, already at this old epoch some 

 kind of phsenogamous angiosperms, first representatives of the conifers and the 

 Cycadece had their existence ; for remains of Cordaites have been found in the 

 Lower Devonian, especially in Canada. These Cordaites, like the ferns, the Lycopods 

 and the Equisetacece, were plants of various size; either small, floating, bushy, or large 

 trees. Their stems or trunks were composed of a woody cylinder, the wood being- 

 disposed in concentrical circles and perforated by pores like that of the conifers, and 

 their fruits, of very diversified forms, had a great analogy to that of the Cycadece, of 

 which they have been considered as the ancestors or prototypes. Hence it is prob- 

 able that from its origin the land vegetation was characterized by the four essential 

 elements which have composed it in the long series of ages and formations from the 

 Silurian to the Cretaceous. 



In the Upper Devonian already, numerous species of ferns, some of them 

 tree ferns, the Lycopodiacece with their generic divisions, Lepidodendron, Uloden- 

 dron, Knorria, Halonia, Lepidophloios, Sigillaria and Stigmaria; the Calamariece as 

 Catamites, Asterophyllites, Annnlaria, Sphenophyllum and the Cordaitece, as Cordaites, 

 are present. Even trunks considered by some authors as referable to the Araucariecf, 

 a family of the conifers, have been found there in England as in America. Except- 

 ing this last kind, all the above genera are more abundantly represented in the 

 Carboniferous, a number of their species passing higher, into the Permian. Here 

 while the large Lycopodiacece and a number of species of ferns lose their predomi- 

 nance and gradually disappear, their place is taken by conifers of a peculiar type, 

 Valchia, Ulmannia, and later in the Trias by Volzia and Albertia, all, like the Arauca- 

 riece, of the order of the Abietacece. It is there also that the Cordaitece give place 

 to tree Cycadece, which gradually become predominant together with the ferns and 

 the conifers. In the Keuper. the Calamariece still represented by gigantic Calamites, 

 become somewhat modified in their conformation, the articulations of their stems 

 becoming like those of the genus Equisetum, surrounded by sheaths instead of whorls 

 of separated narrow leaflets. But in the Lias, the Calamariece and the arborescent 

 ferns become, like the Lycopodiacece. mostly reduced to herbaceous plants ; and in 

 the Oolyte, the Jurassic, the Wealden. even the Lower Cretaceous, the whole vege- 

 tation, though modified in its aspect and its forms, is still composed of acrogenous 

 and gymnospermous plants with a few monocotyledons of as yet uncertain affinity. 



Still in the Cretaceous, but near its base in Europe, the vegetable remains 

 attest the persistence of that peculiar and uniform vegetation which has inhabited 

 the land during such a long series of geological periods. For example, in the 

 Vernsdorf schists of North Germany, which by their fauna and their geological stage 



