66 UNITED STATES GEOLOGICAL SUEVEY— TERTIARY FLORA. 



size of the leaves to Snlvinia Mildeana, Gopp., as described by Hear (Bait. 

 Flor., p. 17, plate iii, figs. 1 and 2), differing by broader, less distinct, areolae, 

 the absence of a dividing middle nerve or free line, and the base narrowed 

 to a short pedicel. These differences may be due to the unripe state of the 

 plants, perhaps, when imbedded in the matrix where they are preserved. 

 This seems indicated by the smaller size of one of the leaves and the 

 indistinct areolation. Salvinia natans of the present flora has its leaves 

 scarcely nerved in the middle before they attain their full development, or 

 rather the middle nerve becomes visible only upon full-grown leaves. The 

 more marked difference is in the form of the base of the leaves, which is cor- 

 date in the European species. This character is preserved also in the figures 

 and description of )S. Mildeana, Ung. (Sill., p. 5, pi. i, figs. 7, 8). 

 Habitat. — Point of Rocks ( Wm. Clehuni). 



CALAMARI^. 



Like the Ferns, this class of plants had its predominance in the Car- 

 boniferous period, when it was especially represented by the Calamites and 

 Equisetites, already present in the Devonian ; plants of large size, reaching 

 the dimensions of trees, with trunks sometimes twenty centimeters in diam- 

 eter. The section of the Equisetacece has followed after the disappearance 

 of the Calamites, from the Keuper to the present time. In the old forma- 

 tions, these plants are still of large size, but from the Tertiary we find the 

 genus as it is now, with species whose stems are rarely more than two centi- 

 meters thick. They are about equally distributed from the Triassic upward. 

 This formation has ten species, the Jurassic has eight, the Wealden five ; 

 from the Cretaceous, two only are known, and fifteen from the Tertiary. 

 The Schizoneura, another genus of this order of plants, is Triassic mostly, 

 one species only being known from the Oolite. Phillotheka represents spe- 

 cies found in the Oolite of New Holland. Its place seems intermediate 

 between Calamites and Equisetum. 



In the present vegetation of the world, the group of the Equisetacece. 

 has about twenty-five species known until now. They are distributed in 

 Europe, Asia, and the American continent, which has the largest number, 

 twenty-one species, of which nine belong exclusively to its flora. 



