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 Conifere, 
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, Tazodiwm distichum, is repre- 
sented by an apparently identical form, 7. distichum 
miocenum, 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 
