JURASSIC FLORA OF DOUGLAS COUNTY, OREG. 77 



can be said is that the piant may be Heer's fossil. The pinnules are 

 narrowed wedge-shape to the base and seem to have dentate margins. 

 The nerves are composed of a parent nerve, which sends off very ob- 

 liquely lateral nerves. The teeth on the margin seem to be acute and 

 very obliquely placed, being mostly on the anterior margin of the 

 pinnules. 



Fig. 9 represents one of the specimens. Fig. 10 a portion enlarged, 

 and Fig. 11 shows what seems to have been the original form of the 

 pinnules. 



Genus Ti^NIOPTERIS Brongniart. 



Forms like Tseniopteris are more common in the Oregon Jurassic 

 flora than the ferns with smaller pinnules. I shall use the distinction 

 suggested by Nathorst as an essential one between Tseniopteris and the 

 unsegmented Nilsonias that, in shape, so much resemble Tseniopteris. 

 This distinction is that the lamina of Tseniopteris is attached to the 

 side of the midrib and in Nilsonia to the upper surface. This feature 

 •causes a Tseniopteris to show a distinct midrib, whether the upper or 

 the under surface be presented uppermost. In the case of Nilsonia, 

 however, when the upper surface is seen uppermost there is no visible 

 midrib or axis. The nerves belonging to the lamina on opposite sides 

 of the axis meet in a raised cord in the center of the position that would 

 be occupied by the midrib if it were shown. But if the lower surface of a 

 Nilsonia be presented uppermost the axis or midrib is seen, and, therefore, 

 while the absence of a distinct midrib may be taken as showing that the 

 plant is a Nilsonia, yet, in cases where a midrib is shown, one can not be sure 

 that the plant is not a Nilsonia with its under surface presented uppermost. 



There are in the Oregon collection a number of leaves that in their 

 shape are like Tseniopteris. They are, however, never seen with their 

 laminse divided or segmented. They show no midrib, but have their 

 lateral nerves meeting in a raised cord that occupies the central line 

 of the position that would be occupied by the midrib if it were present. 

 These leaves have uniformly in their laminse a thin texture. In some 

 the lateral nerves are always single and show no thickening toward 

 their bases near their insertion on the central cord. Others, with the 

 same unchanged thickness in the lateral nerves, have them rarely forked, 

 but in such way as to show that the essential character here, too, is an 



