Amazon Valley and that of the geologically old plateau 
which gives rise to most of the great southern tributaries 
of the Amazon. This might explain the apparently 
anomalous occurrence in northwesternmost Amazonia 
of rare elements which have their greatest diversification 
in southeastern Brazil; or, as in the case of the concept 
herein described, the close relationship of some species 
known from these two widely separated areas of central 
South America. 
Mount Chiribiquete, the type locality of the Styrax 
which is herein described, has yielded a number of strik- 
ing endemic plants and numerous species which exhibit 
a definite relationship with elements known only from 
Mounts Duida or Roraima. Styrax rigidifolius is one of 
the few species from the shrubby cover of the flat sum- 
mit of Chiribiquete which does not seem to have a close 
ally inthe northern Amazon region or on the Venezuela- 
Guiana land-mass. It isnot common but grows hidden 
with the more abundant Graffenrieda fantastica Schultes, 
& Smith, Micus chiribiquetensis Dugand, Hevea nitida 
Muell.-Arg. var. towicodendroides (Schultes & Vinton) 
Schultes, Roupala saxicola Schultes, Senefeldera chiri- 
biquetensis Schultes & Croizat, Bombax coriaceum Mart. 
& Zuce., Vellozia phantasmagoria Schultes, several Vo- 
chysias, numerous species of Clusza, an interesting Tern- 
stroemia, and an undescribed species of Letgebia. All 
of these plants are completely adapted to xerophytic 
conditions and high radiation, and, in this connection, it 
is interesting to note that Styraw rigidifolius itself has 
unusually rigid-coriaceous leaves with a very strongly 
inrolled margin. 
There has been no species of Styrax recorded from 
Mount Duida. The only member of the genus reported 
from Mount Roraima is Styraxv roraimae Perk., an en- 
demic of this peak; it bears no relationship to Styrax 
[ 827 ] 
