Yol. 69.] THE CLEVELAND DISTRICT OE YORKSHIRE. 241 



The species Nilssonia orientalis was founded by Heer in 1878 

 for specimens from Siberia characterized as follows : — 



' N. foliis integris vel vario modo incisis, nervis numerosissimis subtilissimis,. 

 costulis omnino obsoletis.' (Op. supra cit. vol. v, No. 2, p. 18.) 



Two years later Prof. Nathorst described as N. tenuinervis some 

 linear fronds which often exhibited irregular lobes, and had been 

 found in Yorkshire. Subsequently Prof. Seward adopted this name- 

 for Yorkshire specimens at Manchester and the British Museum. 

 He only found a few specimens in these collections, and noticed that 

 they may be compared with Heer's species. In 1909 Prof. Nathorst 

 re-examined his specimens, came to the conclusion that they 

 showed affinities with the ferns, and proposed for them the new 

 generic name of Nilssoyiiopteris. A further examination of the 

 specimens in the Stockholm Museum, and also of a number of 

 examples collected by myself in Yorkshire, has convinced me that 

 Prof. Nathorst had included some fronds of Tceniopteris vittata 

 among the specimens which he studied, and that it was largely on 

 the evidence derived from the latter that his conclusions were based. 

 It is often very difficult to distinguish between specimens of 

 Tceniopteris and this Nilssonia; but, while the former show the 

 supposed fern-like characters, the latter is a true Nilssonia. I was 

 able to show my preparations to Prof. Nathorst, and he agreed with 

 the conclusion reached. A fuller account of the investigation will 

 be published later, but in the meantime the generic name of 

 Nilssoniopteris must be dropped. 



We may next consider the specific name of the Yorkshire form. 

 The Yorkshire fronds are usually somewhat narrow, about 2 cm. 

 wide, and of considerable length. They have a narrow midrib, 

 rather more than 1 mm. wide, with the lamina continuous above it. 

 In casts of the upper side a sharp narrow ridge is seen, corresponding 

 to a groove on the original leaf. The margin is entire, but the 

 lamina is often irregularly incised into very unequal lobes (see 

 PI. XXIII, fig. 7). It may be undivided for a distance of 4 or 

 5 cm., and then show a few lobes, some of them perhaps only 

 5 mm. wide, above which is another undivided portion. The veins 

 are very fine ; they are given off almost at right angles to the 

 midrib, but curve upwards more or less as they run out to the 

 margin. They are undivided and are crowded together, three of 

 them occurring in each millimetre. Some of the Marske specimens 

 are well preserved in fine-grained ironstone, and show the veins 

 very clearly ; the latter appear much stouter than in the Gristhorpe 

 specimens, but have the same frequency, and probably the stoutness 

 is chiefly due to the mode of preservation. The Marske specimens- 

 often surpass the others in size, some of the fronds having a width 

 of 4-5 cm. 



When these fronds, which are undoubtedly of the typical 

 N -tenuinervis type, are compared with their representatives 

 from other localities, they seem to be quite indistinguishable from 

 Heer's species orientalis. They possess the same crowded fine 



