GEOLOGICAL AND GEOGRAPHICAL DISTRIBUTION 227 



flowers can be made out, even to the inner structure 

 of the large pollen-spores, in which the gametophytic 

 structure is more complicated than in any existing 

 Gymnosperms. The pollen-spores have even been 

 detected in the receptacle above the opening of the 

 ovule. 



It is doubtful whether any true Conifers existed prior 

 to the Permian formations, where forms allied to living 

 genera occur, but no existing genera, except Gingko, 

 which probably should be removed from the Coniferse, 

 occur until the Mesozoic, when a number of the living 

 types are encountered. In the later Mesozoic, especially 

 the Cretaceous, and in the early Tertiary formations, 

 they become abundant and characteristic fossils, some 

 of which are scarcely distinguishable from living forms. 

 Most of the existing genera are represented in the 

 Cretaceous rocks, and in some cases even living species 

 can be recognized. Thus the bald cypress of the 

 southern United States, Taxodium distichum, is repre- 

 sented by an apparently identical form, T. distichum 

 mioaenum, which is a common and widespread fossil 

 of the later Miocene and early Pliocene rocks, having 

 been evidently far more widely distributed than at 

 present, as is also the case with the related genera 

 Glyptostrobus and Sequoia. The latter genus is at 

 present reduced to two species, the coast redwood and 

 the giant tree, confined to the mountains of California, 

 but during early Tertiary times both of these, as well 

 as many others, were common trees of nearly the whole 

 northern hemisphere. 



The pines and firs appear first in the middle of the 

 Mesozoic, becoming later more abundant, and holding 



