404 Bacon on Polythalamia, &c. 
all parts of the globe, and often making up a large part of the 
rock. But they occur also in the tertiary strata, though there 
accompanied by various larger forms belonging to genera 
peculiar to that period. Whether the fragments mentioned 
as present in the sand are such as would allow us to refer it 
to any of the divisions of the tertiary, my slight acquaintance 
with the subject does not enable me to decide. The poly- 
thalamia of the chalk are not extinct species, having been 
found living in our present seas. It has occurred to me that 
these in the sand might possibly be recent; but the circum- 
stance that many of them have their cells completely filled 
with calcareous matter renders it very unlikely. The major- 
ity, however, are empty ; but such is also the case with those 
which I have examined from both the secondary and tertiary 
strata. From Mr. Weaver's Abstract of Ehrenberg’s Memoir 
on the Microscopical Structure of Chalk, &c., published in 
the Annals of Natural History, for 1841, I find that Ehren- 
berg is aware ‘of the existence of polythalamia in the sand of 
the Libyan Desert, but no mention is made of any of the 
species present. 
Through the kindness of President Hitchcock, of Amherst 
College, I have been favored with a portion of the specimen 
of Sand from the Desert of Arabia, described by him in an 
article on the Geology of Western Asia, contained in the 
Transactions of the American Geological Association, pa? 
352. Its general appearance is similar to that of the spect 
men from the Sahara Desert, except that the eye can detect 
no caleareous particles in it. The quartz grains are, however; 
larger, (averaging 4, of an inch,) and appear more transparent 
and of a somewhat deeper color. 'This specimen proved e 
be entirely free from polythalamia, consisting merely of red- 
dish-yellow quartz grains. 
