xxxix] NiLSSoiinA 575 



Nilssonia orientalis Heer. 



In this Jurassic and Lower Cretaceous species, founded on 

 material from Jurassic rocks in Siberia^, the lamina is generally 

 entire. The fronds exhibit a wide range in size and shape ; they 

 may have the form of very narrow linear leaves barely 1 cm. 

 across or may reach a breadth of nearly 10 cm. The leaves from 

 Middle Jurassic British strata named by Nathorst N. tenuinervis^ 

 are probably identical with Heer's type, and N. Johnstrupi^ 

 Hear from the Lower Cretaceous of Greenland may also be referred 

 to N. orientalis. 



The veins are at right-angles or obhque to the rachis and are 

 very numerous, three or more in a breadth of 1 mm.* A species 

 recently described by Halle ^ from Graham Land as N. taenio- 

 pteroides agrees closely in habit with N. orientalis : in the southern 

 form the frond may have a length of 40 — 50 cm. and a breadth 

 of 3 cm. The entire lamina of N. taeniopteroides tapers gradually 

 towards the base and is more rapidly narrowed in the distal region ; 

 the petiole is at least 6 — 7 cm. long: the simple or branched veins, 

 15 — 20 in 1 cm., are given off from the axis at a wide angle. The 

 more crowded veins in N. orientalis constitute one of the few 

 features in which it differs from Halle's species, but the fact that 

 the degree of closeness of the veins is inconstant within the same 

 species renders this distinction of doubtful value. HaUe compares 

 his. species also with N. densinervis (Font.) originally described 

 as Platypterigium densinerve from the Potomac beds* and after- 

 wards transferred by Berry' to Nilssonia and considered by him 

 to be identical with Fontaine's Platypterigium Rogersianum. An 

 examination of some of Fontaine's specimens in the Washington 

 Museum led me to refer the fronds to Nilssonia^. 



It is impossible to speak with confidence as to the absolute 

 specific identity of N. orientalis Heer, N. Johnstrupi Heer, 

 N. taeniopteroides Halle, and some similar forms ; but it is clear 

 that the linear fronds of this type characterised by an entire 



' Heer (78) ii p. 18, PI. iv. figs. 5—9. 



■' Seward (00) B. p. 230, fig. 41. * Heer (82) B. PI. vi. 



* For figures and references, see Seward (11^) p. 695; Thomas (11) p. 86. 

 ■i HaUe (13=) p. 47, Pis. v., vi. 



* Fontaine (89) B. Pis. xxx. — ^xxxv. 



' Berry (11) p. 362, Pis. Lvn., Lvni. " Seward (00) B. p. 224. 



