GEOLOGICAL HISTORY OF GYMNOSPERMS 37 



branching, and sporophyll structure. Their stems were pre- 

 vaihngly slender. Their fronds, of which innumerable genera 

 and species have been described from all parts of the world, are 

 among others the following types: Ptilophyllum, Zamites, 

 Otozamites, Sphenoza mites, Giossozamites , Sewardia, Dictyoza- 

 mites, some Podozamites, Wielandiella, Nilsonia, Pterophyllum, 

 Dioonites, etc. 



The fructifications, which were the basis for the establishment 

 of the genus Williamsonia long before their botanical affinity 

 was known, were borne on an extended bracteate peduncle 

 which is interpreted as a continuation of the slender stems but 

 which may sometimes have been a lateral branch. These 

 sporophylls consist of an ovulate receptacle much like that of 

 Cycadeoidea surrounded by a whorl of microsporophylls that 

 are much more reduced than in Cycadeoidea. These form a 

 fairly well defined reduction series illustrated by the genera 

 Cycadocephalus, Weltrichia, Williamsonia, and Wielandiella. 

 Some are unisporangiate and are thought to be whorls from the 

 cones of either monoecious or dioecious forms. For the details 

 of the structure and relationships of these forms the reader must 

 be referred to the recent literature on the subject. 



The evidence for the existence of Mesozoic Cycadales of the 

 modern type is decisive enough, but since the evidence of the fossil 

 fronds has been invalidated by the discovery that they may rep- 

 resent any of the cycadophyte orders it seems probable that the 

 Cycadales themselves were never an extensive group. The evi- 

 dence for their existence in the Mesozoic rests on fossil carpellary 

 leaves of the Cycas type in the lower Jurassic and on Zamia-like 

 cones in the upper Triassic. 



Coniferophyta 



The Ginkgoales were abundant during the Mesozoic, but the 

 differentiations were mainly of specific rank and very few varia- 

 tions of even generic rank occur. The two main types known — 

 Ginkgo and Baiera — -were much alike in fructifications and dif- 

 fered chiefly in foliage, the former with long petioles and the 



