L. E. Robinson and J. Davidson 
37 
with the sheaths by a thin membranous invagination of the latter. 
This inner sheath (s.ch'.) is formed by the inflected distal margin of the 
outer cheliceral sheath which is invaginated between the outer sheath 
and the shaft, and passes backwards as far as the base of the latter, 
where it is again reflected forwards and continues directly into the 
margin of the expanded base of the cheliceral shaft. The relations of 
these parts are clearly shown in the longitudinal section of the capi- 
tulum (Text-fig. 2) and in the transverse sections of the capitulum 
(PI. V, figs. 11-20). Posterior to the buccal cavity, the ventral portion 
of the inner membranous sheath is fused, to a limited extent, with an 
internal chitinous structure, the suh-cheliceral plate (see PI. V, figs. 
19 and 20, sub.ck.p., also Text-fig. 2), but becomes free again in its 
posterior portion, where it forms the loose reflected fold which continues 
directly into the posterior extremity of the cheliceral shaft, thus 
allowing the necessary freedom of movement for the protrusion of the 
chelicerae. 
Rotation of the chelicerae within their sheaths is prevented by the 
fact that the mesial face of the expanded basal portion of each is flat¬ 
tened, and the two plane surfaces are permanently maintained in close 
apposition (see PI. V, figs. 19 and 20). Unlike the Ixodidae, the dorsal 
surfaces of the cheliceral sheaths are not “ shagreened.” The separation 
of the two sheaths commences on the ventral surface of the common 
sheath (see p. 36) as a median fold of chitin, which commences 
posteriorly at the level of the opening in the floor of the buccal cavity 
leading to the pharynx (see PI. V, fig. lo). As it runs forwards, this 
median fold dips more and more deeply between the two cheliceral 
shafts (see figs. 14 and 13) and finally fuses with the dorsal wall of the 
basis capituli (fig. 13) which divides at this point (fig. 12) and thus com¬ 
pletes the separate sheaths (figs. 12 and 11). Owing to the fact that the 
cheliceral sheath extends further forwards on the dorsal side of the 
chelicera, only this portion figures in the first section (fig. 11) of the 
series represented on Plate V, which passes through the extreme portion 
of the sheath. 
The chelicerae are retracted by a pair of stout muscles which are 
inserted into the posterior extremities of the shafts. These retractor 
muscles pass directly backwards and upwards, and are attached to the 
under surface of the dorsal cuticle, where their insertions are indicated 
externally by the two large obliquely placed oval discs situated on 
either side of the median line of the dorsum at the posterior limit of 
the anterior fourth of the body length (see PI. II, fig. 3). Protrusion 
