ratio for teosinte, although within the upper level of the 
long axis dimension of teosinte. The existence of these 
large fossil grass-grains at these great depths calls for an 
explanation and the following possibilities must be con- 
sidered. 
ay 
4. 
The fossil grains are those of a wild grass, not re- 
lated to maize or its relatives. This possibility seems 
quite remote, since, except for the cultivated cereals 
of the Old World, no grass pollen approaching this 
size is known. 
The grains represent contamination occurring in the 
laboratory. This possibility has been eliminated 
from consideration by re-examination and re-isola- 
tion of grains from additional samples of the core 
sediments. It may also be ruled out on the grounds 
of the physical, chemical and optical properties of 
the fossil grains when compared to modern maize 
pollen. 
The grains represent contamination which occurred 
in Mexico during the core drilling (a) either as at- 
mospheric contaminants or (b) as stratigraphic con- 
taminants during the drilling operations. Possibil- 
ity 8a can be excluded on the same grounds as 
possibility 2. Possibility 8b can logically be ex- 
cluded on the ground that if the large fossil grains 
were carried down from the upper 6 meter levels, 
they should be found at intermediate depths. How- 
ever, the large grains have not been found between 
the 6 meter level and the 69 meter level, with the 
sole exception of the one Tripsacum grain referred 
to previously. 
The grains are those of either Tripsacum or teosinte 
[ 237 ] 
