78 MESOZOIC FLORAS OF UNITED STATES. 



iinforked condition. .Xotwillislanding the entii'c nature of tlie laminae 

 of these leaves I shall class them as Xilsonia. 



There are other ta>niopteroid forms in this floia tliat always show 

 strong prominent midril)S. They have a ver>- thick leaf-substance 

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

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

 thickened, near their insertions. These nerves go off approximately 

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

 go off obliquely and curve in a characteristic wa>- 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 niidiib of Tjeniopteris 

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

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

 uppermost? 



The leaves that I group as Ta^niopteris very often occur .with a 

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

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

 fragments. 



T^NIOPTERIS OROVILLENSIS FoiltaiuC. 



\ 



PI. XII, Figs. 12-17. 



1896. Txniopieris ororillensis Font.: Am. Jouni. Sci.. 4th sor.. Vol. II, p. 274 



(nomen) . 

 1900. Tseniopteris orotnllensis Font.: Twentieth Ann. Rep. V. S. Geol. Surv., Pt. 



II, 1898-99, p. 348, ])1. hi, figs. 2-4. 



This Ta^niopteris, so al:)undant in the Oroville Jurassic flora, is 

 found also at some of the Oregon localities. It is not so al^undant 

 and widely diffused as some of the other species of Tfeniopteris. The 

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

 tioned above for Xilsonia, 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 Xilsonias, 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. 



