108 A. S. Packard—New North American Phyllopoda. 
The whirling sand storms of the desert are probabl torna- _ 
does without the accompaniment of clouds with thunder and 
lightning. .The great desert of Africa presents an uninte: | 
rupted surface of sand to the sun’s rays. The lower strata over 
its broad expanse are heated uniformly, and this first requisite | 
is answered. But the dryness of the air fixes the dew point at | 
an extremely low degree, and there can consequently be no — 
condensation or cloud. Yet the vortices may form as in other 
localities, and vast quantities of sand take the place of cloud. | 
y information is too limited, however, to justify any post © 
tive theory in regard to these desert storms. 1 merely conjet | 
ture that they are formed like the tornadoes of our States, but | 
that, unlike them, they are generated from direct heat rays _ 
absorbed by the surfaces immediately under them, and there ~ 
fore appear generally in the daytime, and in the summer months, | 
as well as at any other period. : 
a . 
Art. XIV.—Preliminary Notice of New North American Phyl , 
lopoda; by A. S. Pacxarp, Jr., M.D. ‘ 
7 APODID#. 
The known species of Apus may be for convenience divided 
into three sections, im part by the length of 
