xxxvrr] whjuamsonia 423 



and the Aagiosperms, Saporta's words are still pertinent. 

 Wieland's discoveries in Mexico^ have furnished additional 

 evidence of the wide geographical distribution of the Williamsonia 

 type of flower in the Jurassic period, and it may be confidently 

 asserted that the Bennettitales, including both Williamsonia and 

 Cycadeoidea, occupied a dominant position in the floras of the 

 world during the stage of plant-development immediately pre- 

 ceeding the evolution and rapid spread of the Angiosperms, the 

 present dominant class. 



There has been considerable uncertainty among authors with 

 regard to the appUcation of the name Williamsonia. In former 

 accounts of the genus the name was employed by me both for 

 leaves and flowers on the ground that WiUiamson was correct in 

 his opinion as to the connexion between Williamsonia gigas and 

 Zamites gigas. The tjrpe of frond to which the latter term is 

 applied is by no means tmcommon in Jurassic strata though it is 

 not always associated with flowers, and the use of the generic 

 term Williamsonia is therefore not invariably justifiable. Nathorst^ 

 has recently reiterated his opinion that it is inadvisable to employ 

 the name Williamsonia except for the flowers or the complete 

 plant and strongly urges palaeobotanists to retain the provisional 

 genus Zamites when the fronds only are in question. While 

 agreeing with the contention that the greatest care should be 

 exercised to avoid the use of generic names implying a correlation 

 of vegetative and reproductive organs that rests on any evidence 

 short of demonstration, it may be suggested that the better plan 

 is to add the name Williamsonia in parentheses after Zamites or 

 Ptilophyllum in cases where there is no reason to doubt that the 

 fronds belong to a Williamsonia plant. 



Williamsonia gigas (WiUiamson). 



The species selected for a rather detailed description is stiU 

 imperfectly known, but it is particularly interesting as the type 

 on which the first scientific account of the genus was based. The 

 name Williamsonia gigas is now generally employed for the 

 flowers which bore megasporophylls as the essential organs : 

 they may have been bisporangiate, — a 7iew that seems to me 



1 Wieland (13). 2 Nathorst (13). 



