SEED PLANTS 173 
THE GNETACEZ 
The last order of the Gymnosperms, the Gnetacee, 
are forms familiar only to the botanist, the only exam- 
ples occurring in the United States being a few species 
of Ephedra in the deserts of the Southwest. The other 
two genera are strictly tropical. It is a question how 
closely the three genera are related, as they differ very 
much from one another, as well as from the other Gym- 
nosperms. Some of them show certain analogies with 
the Dicotyledons, and they are sometimes regarded as 
forms connecting the Gymnosperms with the latter. 
Their development is not known with sufficient com- 
pleteness, however, to make this at all certain, and the 
few fossil remains attributed to this order are much too 
imperfect to throw much light upon their geological 
history. 
Fosstn CONIFERS 
Most of the living genera of Conifers are also found 
fossil, and some of them which are now restricted to a 
very limited area were evidently much more widespread 
in earlier geological times. None of the living genera 
can be traced with certainty further back than the earlier 
Mesozoic rocks, although a number of fossils from the 
coal measures have been doubtfully assigned to existing 
‘genera. In the later Mesozoic and early Tertiary rocks, 
however, there are abundant evidences of the existence 
of many living genera, or, in a few instances, even spe- 
cies. A notable case is that of the genus Taxodium, with 
two existing species in the southeastern United States 
and Mexico. Of these, the common bald cypress of the 
