570 CYCADOPHYTAN FRONDS [CH. 



Some of the fronds from the Rhaetic of Franconia described by 

 Schenk as N. polymorpha are examples of N. brevis. In N. poly- 

 morpha the distal ends of the segments are blunter, the lamina 

 is much less folded and when folds or corrugations occur they are 

 less regular than in N. brevis. The epidermal cells are polygonal 

 and have thick, straight, walls: oval or circular papillae occur 

 both on the lamina and rachis. The stomata, confined to the 

 lower surface, are between the veins and each is accompanied by 

 a ring of subsidiary cells (cf. fig. 611, B). N. polymorpha is a 

 member of Rhaetic floras and is recorded also from Liassic rocks 

 at Hor 1 . 



Nilssonia brevis Brongniart. 



This species 2 (fig. 619, C, D), one of those from Hor on which 

 the genus was founded, has been very fully investigated by 

 Nathorst 3 whose researches have cleared up many obscure features. 



FIG. 620. A, B, Nilssonia brevis ; A, section of the frond ; B, section of the lamina 

 parallel to the rachis. C, N. saighensis. (A, B, after Nathorst.) 



The young fronds are circinate like those of Ferns and Cycas. 

 The linear fronds agree generally in habit with those of N. poly- 

 morpha and N. compta, but the lamina is thicker and is characterised 

 by regular transverse corrugations; the veins are very rarely 

 visible except in young leaves which have not reached their full 

 thickness. The fronds may be 50 cm. long and vary in breadth 

 from 1-5 to 12 cm. ; the petiole is very short or the frond may be 



1 For other references see Nathorst (09 2 ) p. 10. 



2 Brongniart (25) p. 218. 



3 Nathorst (09 2 ) p. 12, PI. i. figs. 235, etc. 



