L. E. Robinson and J. Davidson 
41 
margins of which are drawn out into these ridges, is flattened and often 
slightly concave. The second and third articles of the palp {p. ii and 
p. Hi) call for no special remark. They are almost cylindrical, and like 
the basal article, are about as broad as long. The fourth and terminal 
article {p. iv) is appreciably smaller in diameter than the others. It 
tapers slightly, and terminates in a truncated, flattened, or even 
somewhat concave exti’emity, upon which is borne a dense cluster of 
short tactile hairs. 
A comparison of the palp of Argas persicus with that of any of the 
Ixodid ticks, shows at once, that in the former more of the pi’imitive 
characters of the Arachnid “ pedipalp ” are retained. In the Ixodidae, 
the palps have lost a good deal of their mobility, being freely articulated 
at the place of insertion into the basis capituli only, and the terminal 
article is reduced to a diminutive tactile papilla situated in a deep fossa 
on the ventral surface of the third article. The internal face of the palp, 
comprising the first, second and third articles, is always hollowed out 
in such a manner that in the state of adduction the palps partially 
ensheath the lateral surfaces of the median structures of the capitulum— 
the chelicerae and the hypostome. In Argas persicus the ridges of the 
basal article, described above, in all probability signify an approach to 
this condition, but the rudimentary attempt is limited to the single 
article. 
A very few hairs appear on the ventral surfaces of the palp, but as 
previously mentioned the dorsal surfaces are scattered with numerous 
anteriorly directed hairs, which, in conjunction with corresponding hairs 
on the roof of the camerostome and the dorsal surface of the basis 
capituli, serve the purpose of excluding dirt from the camerostomal 
depression. 
The Hypostome. 
The hypostome (h.) is a median prolongation of the ventral portion of 
the basis capituli. Proximally it forms the floor of the buccal cavity, 
and its distal portion is apposed to the ventral surfaces of the chelicerae, 
the three structures collectively forming a piercing and sucking organ 
or proboscis. The base of the hypostome is wide, but the lateral 
margins rapidly converge and follow a nearly parallel course from the 
middle of the organ to its distal extremity. With the exception of the 
tip, which is flattened, the ventral surface is strongly convex from side 
to side, and its distal half is armed with stout, recurved denticles which 
are disposed in more or less regular longitudinal rows. The arrangement 
