402 Bacon on Polythalamia 
ART. XXX.—POLYTHALAMIA IN SAND FROM THE SAHARA 
DESERT. By Joun Bacon, Jr., M. D 
Tunere is in the Cabinet of our Society a specimen of Sand 
from; the Desert of Sahara, which I find to be partly com- 
posed'of microscopic Polythalamian shells. I am unable to 
state from what part of this widely extended desert it was ob- 
tained. The society having committed this specimen to Dr. 
Charles T. Jackson for chemical analysis, a portion was kindly 
placed by him in my hands for microscopic examination. 
Under the microscope, the sand is seen to consist mainly 
of irregular quartzose grains, of a reddish-yellow color, and a 
pretty uniform size, ranging between $ and jj of an inch. 
Wefcanfreadily understand the facility with which the wind 
raises in clouds and conveys to great distances sand so fine as 
this. These grains are more or less rounded on the angles 
and edges. A few minute quartz crystals are also met with, 
presenting the usual six-sided prism with pyramidal termina- 
tions, and rounded in the same manner. 
Among the translucent. particles of quartz, a considerable 
number of rounded opake grains, of a white color, may 5e 
perceived by the unassisted eye. Some of these were sub- 
mitted, under the microscope, to chemical tests, and proved 
to be calcareous. In order to examine them microscopically, 
the sand was put up in Canada balsam, on slips of glass, and 
covered by thin mica or glass. Being now viewed by trans- 
mitted light, with a moderate power, the white grains (to 
which the balsam communicated a degree of transparency) 
were at once seen to be Polythalamian shells; generally muc 
_ broken, and presenting a worn and rounded exterior ; 8° that 
nothing satisfactory could be made out when they were exam- 
ined by reflected light. 
Between one and two hundred specimens were 
from a considerable portion of the sand, but only six o 
species could be made out. In the figures accompanying 
x 
obtained 
or seven 
this 
