CUTICLES OF CYCADEAN PEONDS. 



183 



I. 



on either side of the guard-cells are two well-marked subsidiary cells. The figure 

 further shows that the guard-cells of adjacent stomata w^ere approximately parallel to 

 each other, and that the inclined guard-cells became more vertical at the poles and 

 their upper or outer thickening became less. In figure 8 of the same plate another 

 stoma is shown cut through more or less longitudinally, but the section has not here 

 passed through the centre. No stomata were seen in surface view, but in a later paper * 

 surface views are given of stomata from the interseminal scales of the same specimen, 

 which had been obtained by collodion casts. Though the details are not very clearly 

 shown, it is evident that these stomata were of the same type as those which are 

 described here, the guard-cells and subsidiary cells of some of them being quite charac- 

 teristic. The structure of these stomata in Lignier's species of JSemietHtes, a genus 

 closely allied to our Yorkshire WilUamsonia, not only provides an interesting and very 

 close comparison with the modern Bowenia type, but also goes some way towards 

 demonstrating that the interpretation which we have given above, of the meaning of 

 the surface views of stomata in this group, is correct. 



We have dealt at some length with the structure of these Ftilophyllum stomata 

 because the form of these organs is remarkably constant throughout the group of fronds 

 which we have assigned to the Bennettitales. Moreover, their appearance is so complex 

 that at first sight it is very difficult to understand them ; no previous observers seem 

 to have made any attempt to grapple with the problem, but have merely spoken of the 

 thickening patches as the guard-cells. 



Ptilophyllum sp.f 



Typical examples of Ftilophyllum pecten, from the Lower Estuarine beds of 

 Whitby and Marske, show perfectly smooth cuticles with very few hair-scars. In the 

 larger fronds, however, from the same beds, a large number of very characteristic 

 structures are seen, vrhich are also present on the fronds of the small pecten-form which 

 occur plentifully in the Cloughton Wyke beds of Middle Estuarine age. 



These structures are show^n in PL 20. fig- 4. They may be described as small annulate 

 projections from the surface of the cuticle '03 to -04 mm. in diameter, occurring in 

 numerous regular rows running parallel to the length of the pinna. The rows are very 

 close together, being often sex)arated by a space less than their individual width. The 

 annulate structures of the same row are frequently contiguous and are sometimes 

 unevenly thickened, being broader and darker laterally and thinner at the points next 

 to the adjacent rings, so that an appearance is produced of pairs of crescentic patches. 

 They are so numerous as to obscure entirely the outlines of the epidermal cells and 



make the stomata very indistinct. 



We may regard these structures as the bases of hairs, with which the lower surface of 

 the frond was covered. Scattered hair-scars occur on the ordinary types of frond, wliich 



♦ Ligiiier (1912), p. 426, fig. 1. 



t This species will later be described as Ptilophyllum Tiirsatutn ; a fig 

 the ♦ Fossil Flora of the Marske Quarry,' text-fig. 3 b. Thomas (1913). 



