XXXIX] NILSSONTA 569 



Ginkgoales though without any convincing evidence to support 

 such connexion. Nothing is known of the stems : the occasional 

 occurrence of leaves converging towards a common support 

 points to a Cycadean habit. It is possible, as Nathorst suggests, 

 that the Rhaetic species Bucklandia Saportana 1 may be the stem 

 of a Nilssonia. 



Nilssonia is especially characteristic of Jurassic and Rhaetic 

 floras ; it occurs also in Triassic beds and extends into Cretaceous 

 floras. 



Nathorst instituted the genus Nilssoniopteris 2 for some speci- 

 mens from the Yorkshire coast which he believed to be examples 

 of Nilssonia tenuinervis on the ground that the veins are dichoto- 

 mously branched and the epidermal cells have sinuous walls. 

 Mr Hamshaw Thomas 3 has, however, shown by an examination 

 of the specimens in the Stockholm Museum that they belong to 

 Taeniopteris vittata, and Prof. Nathorst agrees with this conclusion. 

 The name Nilssoniopteris must therefore be abandoned. Miquel 4 

 proposed the name Hisingera for some Nilssonia fronds, but it 

 has not been adopted. 



Nilssonia polymorpha Schenk. 



Linear fronds varying considerably in breadth and in the degree 

 of dissection of the lamina, which may be entire; the margin 

 may show broad and shallow crenulations or there may be a 

 few narrow and deep sinuses cutting the otherwise entire lamina 

 into long and narrow segments. More usually the lamina is 

 divided to the rachis into numerous truncate segments traversed 

 by parallel, simple, veins extending from a narrow groove in the 

 middle of the rachis on the upper surface of the frond (fig. 619, E). 

 The veins run in very narrow grooves in the generally flat but 

 occasionally corrugated lamina 5 . 



This species agrees very closely in habit with N. compta and 

 N. brevis and, as Nathorst points out, it is in some cases almost 

 impossible to distinguish Nilssonia polymorpha from N. brevis. 



1 Nathorst (86) PL xvm. fig. 5. 2 Ibid. (09 2 ) p. 28. 



3 Thomas (13 2 ) p. 241; Thomas and Bancroft (13) p. 193. 



4 Miquel (42) p. 61. 



5 Schenk (67) A. p. 127, Pis. xxix., xxx.; Nathorst (09 2 ) p. 10, PL v. figs. 

 913, etc. 



