78 MESOZOIC FLORAS OF UNITED STATES. 



unforked condition. Notwithstanding the entire nature of the laminae 

 of these leaves I shall class them as Nilsonia. 



There are other tseniopteroid forms in this flora that always show 

 strong prominent midribs. They have a very thick leaf-substance 

 that is often slickensided from the creep of the rock along their surface. 

 The lateral nerves are, as a rule, forked, and are much stronger, or 

 thickened, near their insertions. These nerves go off approximate^ 

 or quite at right angles, and are not curved, while in the Xilsonias they 

 go off obliquel}^ and curve in a characteristic way to meet the margin. 

 There is one exception to this character found in the form Tmiiopteris 

 orovillensis, to be noted further on. This has the midrib of Tseniopteris 

 and the ateral nervation like that of the Nilsonias. The question 

 might arise, Is not this really a Nilsonia with its under surface presented 

 uppermost? 



The leaves that I group as Tfeniopteris verj- often occur with a 

 number together in the same hand specimen of rock, as if they grew 

 in clusters. Those regarded as Nilsonia are usually found in isolated 

 fragments. 



T^NioPTERis OROVILLENSIS Fontaine. 



PI. XII, Figs. 12-17. 



1896. Txniofteris orovillensis Font.: Am. Journ. Sci., 4th ser., Vol. II, p. 274 



(nomen) . 

 1900. Tieniopteris orovillensis Font.: Twentieth Ann. Rep. U. S. Geol. Surv., Pt. 



II, 1898-99, p. 348, pi. lii, figs. 2-4. 



This Tseniopteris, so abundant in the Oroville Jurassic flora, is 

 found also at some of the Oregon localities. It is not so abundant 

 and widely diffused as- some of the other species of Tseniopteris. The 

 nerves are fine but very distinct. They have the character, as men- 

 tioned above for Nilsonia, of being always single, not varying in strength 

 from their insertions to their ends, and of going off at less than a right 

 angle, and of curving in their course to meet the margin. The leaves, 

 however, have a much thicker and more coriaceous texture than those 

 regarded as Nilsonias, and the midrib is strong. The size of these 

 leaves varies a good deal. The most common forms have the sizes 

 seen in the Oroville specimens, but occasionally fragments are found 

 indicating a somewhat wider leaf than any seen in the Oroville plants. 



