696 PROFESSOR A. C. SEWARD ON 



off from the median line of the lamina at almost a right-angle. The broadest leaf is 

 that represented in the photo. 46, PI. X., taken from the specimen figured by Hugh 

 Miller with other Cycadean fronds as Zamia. The lamina is torn in several places 

 parallel to the veins, but the leaf was no doubt originally entire. In the Miller collec- 

 tion is another specimen of the same type, 14"5 cm. long, with a rachis 7 mm. in breadth 

 exposed by the removal of half the lamina; the portion of the lamina preserved is 2 5 

 cm. broad ; there are 6-7 veins per millimetre. The incomplete leaf shown in fig. 60 

 illustrates the broad form of rachis, which in more complete specimens is hidden by the 

 lamina. The tapered base of a frond is seen in fig. 64, and fig. 63 represents the 

 bluntly rounded apex of a narrow leaf in which the rachis is shown as a groove. In 

 the narrow fragment seen in fig. 65 the median line projects as a narrow ridge from 

 which the veins arise almost at right-angles. Better specimens are represented in figs. 

 34, 42, PL IX. ; in the smaller example seen in fig. 40 the lamina is traversed by fairly 

 prominent ribs about 1 mm. apart : it is possible that this is a piece of a distinct species, 

 but, on the other hand, the apparently coarser venation may be an accident of preserva- 

 tion. It is easy to confuse veins with ribs due to the presence of hypodermal strands 

 of strengthening tissue. A petrified specimen of N. orientalis of Cretaceous age from 

 Japan has recently been described by Miss Stopes * in which stereome strands occur 

 below the epidermis. A comparison of the Culgower material with the species from 

 the Yorkshire coast to which Nathorst gave the name N. tenuinervis led me to regard 

 the two sets of specimens as specifically identical ; but it would appear from a recent 

 description by Nathorst of a leaf of N. tenuinervis t from a Yorkshire locality that 

 the secondary veins are occasionally forked and that the cuticular structure differs from 

 that of true Nilssonia fronds. The discovery of these differences led Nathorst to 

 institute a new genus Nilssoniopteris for the English type. In some impressions of the 

 Yorkshire plant which I have examined I have failed to detect any dichotomously 

 branched veins ; it is possible, as Nathorst suggests, that some of the specimens 

 referred to Nilssonia tenuinervis are true Nilssonias, though this is hardly likely. In 

 describing some specimens from the Jurassic of Oregon identified as N. orientalis, J 

 Fontaine speaks of the secondary veins being rarely forked, a fact which suggests the 

 possibility of identity with Nilssoniopteris of Nathorst. Nilssonia orientalis is a widely 

 spread Jurassic type, and it is probable that the plant described by Heer from the 

 Lower Cretaceous of Greenland as N. Johnstruppi § is identical with the Jurassic species. 

 From Jurassic strata N. orientalis || is recorded by Heer from Siberia, by Yabe from 

 KoreaJ by Fontaine from Oregon, ## by Nathorst and Yokoyama from beds probably 

 of Wealden age in Japan, ft and by myself from Jurassic rocks of the Caucasus. A 

 fragment figured by Nathorst from Upper Jurassic beds in Spitzbergen as N. cj. 

 N. orientalis || is probably referable to this species. 



* Stopes (10). t Nathorst (09), pi. vi. figs. 23, 24. J Ward (05), pi. xvi. 



§ Heer (82), pi. vi. || Heer (78), pi. v. IT Yabe (05), pi. hi. 



** Ward (05), pi. xvi. tt Nathorst (90) ; Yokoyama (89). J| Nathorst (97), pi. i. fig- 18. 



