L. E. Robinson and J. Davidson 
39 
cheliceral shaft, which forms a broad articulatory surface on which the 
entire digit rocks. On the dorso-lateral surface, a chitinous outgrowth, 
the dorsal process (d.p.), appears. This.is a crescentic structure, the 
two pointed cusps of which protrude in a lateral direction. The ventro¬ 
lateral face of the internal article is excavated about the middle of its 
length in the form of a deep fossa into which the base of the external 
article {e. a.) is articulated. The external article as seen in profile 
(i.e. from the dorsal or ventral aspect) presents a triangular outline, the 
side of the triangle being produced laterally in a series of curved, 
reflexed cusps, usually three in number, which increase in size from 
before backwards. The form of these cusps, as well as their number, is 
apt to vary in individual specimens, but those illustrated (PL VI, figs. 22 
and 24) show the mo.st frequently occurring types. 
Both articles of the digit, and also the dorsal process, are hollow, 
and their internal cavities communicate with each other, and that of 
the internal article is directly continuous with the internal cavity of the 
cheliceral shaft. The cavity of the internal article also communicates 
with the exterior by means of a large pore (pr.), which penetrates the 
lateral wall of the expanded base a little distance below the insertion 
of the external article. The flexor tendon of the digit divides at the 
base of the internal article into two branches, the larger of which is 
inserted into the base of the internal article and the smaller passes 
upwards to be inserted into the base of the external article. These and 
the extensor tendons of the digit are represented in the sections (PL VI, 
fig. 25, e. and /), where they are distinguished by horizontal line shading. 
The entire digit, with the exception of the pointed cusps and 
cutting-edges of the articles and dorsal process, is surrounded and 
protected by a delicate ensheathing structure which is conveniently 
termed the hood (hd.). The structure and relations of this hood, which 
appear to be less simple than is the case in the Ixodid ticks, are 
impossible to define from the examination of cleared specimens alone, 
on account of the extreme transparency of the parts. For this purpose, 
recourse must be made to serial sections passing through the entire 
structure. It is then seen that the hood is entirely composed of soft 
membranous chitin and is partially derived from the cheliceral shaft, 
and partly from the digit itself. From its relations, it appears reasonable 
to suppose the hood represents an exuberant growth of the soft articu¬ 
latory chitin of the joint between shaft and digit. On the internal side, 
the hood continues the outline of the cheliceral shaft forwards, and 
extends almost to the extremity of the internal article. The distal 
