G. It. Wieland — WilliamHonian Tribe. 445 



II. Williainsonian Culmination and Extinction. 





Trias 

 X 



Rhaet 



Jura 

 L. M. U. 



Cretaceous 

 L. M. U. 



Pterophyllvm . 

 Zi unites.. . 













-> 















-> 



Poduzamites . . 



X 

 X 

 X 

 X 











-> 

 -» 



(?) 





Dioonites . 













WielandieMa .. 















-> 





Nilssonia 







(?) 



Glossozamites . 









-> 



-> 



Sewardia 



Otozamites .... 





X 







X 



-> 





Sphenozamites . 

 Ptilophullum . 





X 

 X 



-> 

 -> 

 -> 





Dictt/ozamites . 















As far as fossil preservation goes, PterophyUum and Zamites 

 are the representative ancient lines, with Sphenozamites as a 

 relative merely characterized by oddity of leaf form. And 

 these old lines pass on to add their quota to the great William- 

 sonian group of the Rhsetic, the period of most rapid evolution. 

 Culmination occurs in the lower Jura; while extinction begins 

 in the middle Jura and progresses steadily to the upper Creta- 

 ceous, where the Cycadeoideas also find their final repesentation 

 in both Europe and America. 



Were tabulation extended to the one hundred or more 

 species referred to the twelve listed genera, the origin and 

 extinction curve might likely remain quite the same as when 

 the genera alone are considered ; though any such fuller elabo- 

 ration of the data of species must remain in abeyance until 

 considerable comparative work with better collected and pre- 

 pared specimens in hand has given generic, to say nothing of 

 specific determination the needed accuracy.* That such ex- 

 tended and accurate study of the species must finally reveal 

 further relationships as well as ecologic factors of deep inter- 

 est, can hardly be doubted. 



Naturally enough, aside from the question of precise specific 

 determination, field work has not reached that advanced stage 

 when much emphasis can be placed on the last reputed occur- 

 rence of even the better known genera. So that it would be 



* For instance, the genus Glossozamites is by no means so isolated as might 

 usually be inferred. Since the superb fronds from the copper mines near 

 Abiquiu, Rio Arriba County, New Mexico, called by Newberry (3) Otozamites 

 Macombii, are very near to Glossozamites zitteli Schenk, being indeed a tran- 

 sitional form between the latter species and Otozamites. 



