cal discovery. ‘The evidence has been secured from a 
series of deep cores taken in Mexico City, in connection 
with extensive studies of the sub-surface. These cores 
have been described in some detail by Sears (1952), and 
Sears and Clisby (1952). 
In the course of analyzing the pollen contents of the 
cores, Mrs. Clisby observed several unusually large grass- 
pollen grains in sediments close to the 70 meter level in 
the Belles Artes boring. The size of the grains (75p to 
135p by acetylation method) at first appeared to preclude 
the reasonable possibility that they were derived from 
native wild grasses then extant in the Valley of Mexico. 
However, careful study of these fossil pollen grains, in- 
cluding detailed comparison with material in the Harvard 
Pollen Collections and in the Botanical Museum, sug- 
gested three possibilities for their botanical affinity, viz: 
Tripsacum, Zea or Euchlaena (teosinte)—all of the tribe 
Tripsaceae. If the grains actually proved to be those of 
Zea Mays, as seems most probable on the basis of their 
size alone, they would extend the fossil record of Indian 
corn far beyond our presently known chronology for the 
existence and dispersal of this remarkable genus of the 
grasses. 
In order to establish a critical basis for identification 
of the fossil pollen, an extensive study was made of the 
size-range exhibited by the pollen of various species of 
Tripsacum and by varieties of maize and teosinte. To 
sustain essential uniformity in the data, all preparations 
of both living and fossil grains were prepared by the same 
technique (modification of Erdtman, 1948), and perma- 
nent slides were prepared with glycerin jelly as a mount- 
ing medium. A total of eight species of Tripsacum, three 
collections of teosinte and fourteen varieties of modern 
maize were chosen for purposes of comparison (‘Table I). 
A total of 84 large grass-pollen grains were ultimately 
[ 230 ] 
