2 THE PALEONTOLOGY OF MINNESOTA. 
(Geological introduction of plants. 
ferns) and the Lycopodiacee. It is very probable that, already at this old epoch some 
kind of phznogamous angiosperms, first representatives of the conifers and the 
Cycadee 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 Equisetacew, 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 Cycadee, 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 Lycopodiacee with their generic divisions, Lepidodendron, Uloden- 
dron, Knorria, Halonia, Lepidophloios, Sigillaria and Stigmaria; the Calamariee as 
Calamites, Asterophyllites, Annularia, Sphenophyllum and the Cordaitew, as Cordaites, 
are present. Even trunks considered by some authors as referable to the Avaucariew, 
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 Lycopodiacew 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- 
riee, of the order of the Abzetacew. It is there also that the Cordarter give place 
to tree Cycadew, which gradually become predominant together with the ferns and 
the conifers. Inthe Keuper, the Calamariew 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 Ca/amariew and the arborescent 
ferns become, like the Lycopodiacew, mostly reduced to herbaceous plants ; and in 
the Odlyte, 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 
