520 CYCADOPHYTAN FRONDS [CH. 



fronds as have been examined are characterised by strongly looped 

 or sinuous walls; the stomata, confined to the lower surface, are 

 roughly circular and the guard-cells are at right- angles to the veins 

 and not appreciably sunk. Fig. 594 represents the appearance 

 of a stoma in surface- view : 'on either side 

 of the central slit-like pore are two elliptical 

 or hemispherical structures ; they are some- 

 what flattened when they abut on the pore, 

 and have rounded ends.... Between these and 

 the subsidiary cells lie two other thickened 

 patches, more or less hemispherical in shape, 

 and apparently overlying the central struc- FIG. 594. Ptilophyllum 

 tures.' On the analogy of similar appear- P ecten '> stoma - < After 



xn j 5? mi i- , Thomas and Bancroft.) 



ances in recent Cycads Mr Thomas 1 interprets 



the two pairs of thickened patches as belonging to the upper and 

 lower sides of the highly inclined guard-cells. This author calls 

 special attention to the abundance on some of the fronds included 

 in the aggregate species P. pecten of regular rows of circular hair- 

 scars preserved as small annulate projections, *03 04 mm. in 

 diameter. A comparison of the cuticles of different forms of 

 Ptilophyllum pecten enabled Thomas to recognise more than one 

 type : for one of these the name Ptilophyllum hirsutum is proposed. 

 It is by such work as this that we may hope to discover differen- 

 tiating characters. 



The different forms of Williamsonia flowers found in association 

 with fronds of the Ptilophyllum habit also point to the inclusion 

 of more than a single species under the group-species P. pecten. 

 As additional evidence is obtained further analysis will be possible, 

 but in dealing with impressions which include specimens transi- 

 tional from one form of frond to another, the most convenient 

 and to my mind the most logical course is to treat a species as 

 an aggregate- or group-species. Some authors believe that the 

 two fronds described by Phillips as Cycadites pecten and C. pecti- 

 noides are distinct species 2 , but there would seem to be no adequate 

 reason for this view. The fronds described by Heer 3 from the 

 Lower Cretaceous of Greenland as species of Zamites and similar 



1 Thomas and Bancroft (13) p. 184. 



8 Phillips (29) A. PI. vn. fig. 22; PL x. fig. 4. 3 Heer (75) n. 



