CYCADACE.E. 1 9 



the rachis, whether towards the tip or the lower part of the leaf, 

 or according to the age of the frond. The terminal pinnae are 

 often strongly decurrent at the hase, whilst the lower segments 

 have a uniform width ; a young frond of Cycas media, Br., 

 shows pinnaa with no indication of tapering towards the rachis, 

 but the older and broader segments are distinctly narrowed. 



Stress is often laid on the form of the pinna apex, whether 

 truncate, acute, etc. In the typical form of Pterophyllum the 

 pinnaa have truncated apices, but specimens are occasionally 

 referred to this common provisional genus in which the apices of 

 the segments are clearly not truncate. Bornemann defines the 

 genus as possessing pinna? which may be either straight at the tip 

 or obliquely truncate, and this wider definition is probably the 

 most satisfactory. In such a specimen as that of Otozamites 

 Gopperttanus (Dunk.), figured in PL I. Fig. 2, some of the 

 pinnas are more or less truncate at the tip, and others regularly 

 acuminate. In the examples of Zamites Buchianus (Ett.) in the 

 British Museum Collection, the variation in the apical terminations 

 of the piuna3 has proved a difficulty, some specimens having 

 gradually tapering segments, and others showing obtusely ter- 

 minated apices, but the occurrence of some intermediate forms 

 throws doubt on the value of such a feature as a leading specific 

 characteristic. 1 In dried fronds of Cycas revoluta, Thunb., it is not 

 uncommonly found that in many of the pinnas the pointed spiny 

 apex has been replaced by a rounded termination, with a slight 

 median depression at the end of the single vein. As a rule, 

 however, the pinnaB of recent fronds maintain a fairly uniform 

 mode of termination in the same species. The venation is not 

 always readily made out even in fairly good specimens ; the thick 

 coriaceous pinnaa of some recent species, with their indistinct veins, 

 prepare us for a similar difficulty in dealing with fossil leaves. 

 It is well known that the lower surface of a pinna often shows 

 very distinct venation, while the veins on the upper surface are 

 quite obscure. In Cycas we have a convenient venation character, 

 which is taken as the essential feature of the fossil genus Cycadites\ 

 but in this case, as we shall see later in describing the genus, 

 frequent mistakes have been made in the determination of speci- 

 mens, which apparently rest on such a readily recognized character 



1 PL III. 



