XXXIX] PTILOPHYLLUM 525 



or to employ that and other specific names as designations of 

 geographical types differing in no clearly defined or constant 

 characters from examples of the English Ptilophyllum pecten. 

 A comparative examination of the cuticular membranes in the 

 comparatively few cases where that is possible would probably 

 furnish a basis for a satisfactory subdivision of the group-species. 

 The names Ptilophyllum Anderssoni, P. boreale, and P. ant- 

 arcticum are retained for certain forms selected from a number 

 of closely allied types partly on the ground that these fronds 

 exhibit some more or less well-marked distinctive characters and 

 in part as a recognition of the existence of geographical forms. 



Ptilophyllum boreale (Heer). 



Heer 1 described numerous well-preserved impressions of pin- 

 nate fronds from the Lower Cretaceous plant-beds of Kome, 

 Greenland, which he referred to several species of Zamites though 

 the differences between them are hardly of specific rank. An 

 inspection of several of the figured specimens in the Stockholm 

 Museum showed that Heer's drawings are in the main accurate. 

 Zamites borealis (fig. 597) is characterised by small linear pinnae 

 attached to the upper face of the rachis, the stoutness of which 

 is a noteworthy feature; the veins are parallel and simple. In 

 the shape of the pinnae, including the base-characters, this species 

 agrees closely with P. pecten. The slightly falcate or straight 

 pinnae are 1-7 2 mm. broad and reach a length of 15 mm. ; 

 in Z. speciosum Heer, probably specifically identical with P. 

 boreale, the pinnae are relatively longer, and in Z. brevipennis 

 Heer they resemble the shorter pinnae of some of the narrow 

 fronds of P. pecten. 



A preparation of the cuticle of a pinna of P. boreale made by 

 Mr Thomas from a specimen in the Stockholm Museum shows 

 a marked difference between the upper and lower epidermis: 

 the cells on the upper surface of the lamina have very thick and 

 sinuous walls precisely as in P. pecten, but the cells are often broader 

 than long and not elongated parallel to the long axis of the pinna ; 

 those of the lower epidermis are thinner and less clearly preserved : 



1 Heer (75) n. p. 66, Pis. xiv., xv. 



