360 Systematic Paleontology 



The genus may be redefined in the following terms : Frond coria- 

 ceous, elongate-lanceolate in outline, entire or commonly more or less 

 deeply pinnatifid by being split, usually to the rachis, into a number of 

 more or less irregular segments which are contiguous, usually broad and 

 truncate. Lamina attached to the upper surface of the rachis, the 

 simple and parallel, equal, lateral veins running almost or quite to the 

 median line. In material showing only the under surface of the fronds, 

 the stout midrib is prominent and unsegmented specimens are 

 scarcely distinguishable from Tceniopteris and allied forms, while the 

 segmented varieties approach Anomozamites or even some species of 

 Pterophyllum in appearance. 



The genus Nilsonia appears in the Triassic and is particularly a 

 Ehaetie and Oolitic type. A number of undoubted species occur, how- 

 ever, in the Lower Cretaceous, no less than seven different species having 

 been recorded from the Lakota, Kootanie, and Shasta deposits. The 

 N'eocomian of Japan furnishes two or three species, while the wide- 

 spread Nilsonia schaumburgensis (Dunker) N'athorst, occurs very abun- 

 dantly at a number of European Wealden localities. The Upper 

 Cretaceous shows a species in the Atane beds of Greenland and one in 

 the Cenomanian _ of Bohemia, while several supposed species have been 

 recorded from Tertiary strata. 



There are two species in the Potomac Group, a lanceolate unsegmented 

 form variously described by Fontaine as Angiopteridium and Sapindopsis 

 and the large and elegant form which this author describes as two 

 species of Plqty pterygium. The latter term was proposed by Schimper, 

 in 1880, as a subgenus of Anomozamites for very large forms of that 

 type. It was subsequently used as a genus by Feistmantel and Fontaine, 

 although this usage seems unwarranted, especially since the Platy ptery- 

 gium forms of AnomozaTYiites are all confined to much older horizons, 

 and the Potomac forms agree in all essential characters with Nilsonia, 

 a relationship suggested by Seward, in 1900, offer examining the 

 material in the U. S. National Museum. As illustrated by Fontaine, 

 the rachis is represented as very wide and the opposite segments are 

 far apart. That the midrib was not wide and flat in life, but prominent 



