OF LOWER GREENS AND PLANTS. 101 



modern and does not in all probability antedate the Tertiary." 

 Furthermore, the assumption is made that "the appearance 

 of marginal ray-tracheids about the beginning of the Tertiary 

 epoch, with the resulting improvement of water-supply, in 

 all probability explains why so comparatively large-leaved a 

 conifer should have been able not only to live on into the 

 modern period, but to flourish as it never had before.''* 



Later, however, another North-American Upper Cretaceous 

 Pityoicylon was discovered and described by Bailey (1911). 

 This specimen shows well-developed ray-tracheids. It is 

 necessary to revise Jeffrey & Chrysler's conclusions, which 

 Bailey does as follows (p. 323) : " The distribution of ray- 

 tracheids in our lignite confirms these writers in their con- 

 clusion that the absence of ray-tracheids is a primitive 

 condition, but shows that these structures were evolved during 

 the latter part of the Cretaceous rather than at the beginning 

 of the Tertiary." 



Further specimens continued to be found in the ClifFwood 

 locality, and Holden (1913 A) describes another species in which 

 ray-tracheids are present (p. 614). Raj'-tracheids, "though 

 rare in the first annual ring of P. protoscleropitys, and their 

 abundance later, seem to indicate that they are a more ancient 

 feature than has been assumed by any of the above-cited 

 investigators. It is probable that they were developed in the 

 Lower Cretaceous if not in the Jurassic.'" 



The above quotations illustrate the uncertainty of conclusions 

 based on single fossils or a small number of finds from isolated 

 localities. The new British species is of Lower Cretaceous 

 (Aptian) age, and shows ray-tracheids of a truly modern type. 

 Consequently, I do not attempt any surmise as to when they 

 were first evolved, but content myself with describing these 

 early specimens, the earliest yet known, in which they exist. 

 The only older known British specimen in which ray-tracheids 

 might be anticipated is " Pinus " Ruffordi, Seward (1896 c), 

 from the Wealden, but they do not occur in this fossil. 



In all the papers describing fossil forms of Pityoxylon, as well 

 as in many descriptions of recent Conifers, theories regarding 

 the origin of the ray-tracheids are elaborated ; but the palaeonto- 

 logical material is still insufficient for the discussion of the 

 subject. 



