116 DR. L. A. BORRADAILE ON THE 
another. The limb is a broad structure, flattened in that direction which is 
morphologically antero-posterior, and widened from side to side. In what 
may be called the normal j^'osition it is turned forwards till its wide plane 
passes the horizontal and slopes a little upward in front. In this position it 
meets its fellow on the middle line, and the two form an operculum which 
almost wholly encloses the mouth-field, abutting behind on the sterna, i'.t the 
sides upon the edge of the carapace, and in front upon the epistome save in 
the middle, where a gap is left through which the gill-stream can flow even 
when the operculum is most tightly closed. Each third maxilliped articu- 
• lates veutrally with the hinder angle of its sternum, and dorsally (anteriorly) 
with the epimerite of its segment. 
The coxa is of oval transverse section. Its proximal rim bears- on the 
under side the knob which makes articulation with the sternum, where it is 
received by a saddle-shaped hollow between two processes. Adjoining this 
is a facet on the median surface of the joint, which works against the 
external (antero-Iateral) face of the sternum. On the dorsal side the rim 
bears a socket which articulates with a correspondingly shaped process of the 
epimerite. On the outer side the coxa has a backward-curving flange, to 
which, b}' a flexible suture, is hinged the epipodite. This organ has a stout 
base, which bears on the dorsal side the small podobranch, and a long, blade- 
like process which, diving under the edge of the branchiostegite, enters the 
gill-chamber and passes between the posterior arthrobranch of its own limb 
and the anterior arthrobranch of the cheliped, to lie between the gills and 
the side of the body. The whole organ, and the flange upon which it is 
borne, is spirally twisted, so that, starling in th'e horizontal plane in which 
the main part of the limb lies below the mouth, it ends in a vertical plane 
against the flank. The flange and the base of the epipodite stand in tliat 
gap, between the anterior face of the coxa of the cheliped and the branchio- 
stegite, which is the anterior inhalent opening of the gill-chamber; and their 
twisted shape bears such a relation to the opening that when the maxillipeds 
are in the normal position they lie across it; and almost but not quite close it, 
but when the maxillipeds are divaricated, the epipodites lie in the midst of 
the opening, with their flat sides parallel {o the stream, to which they offer 
little opposition. The part of the opening which is covered when the 
maxillipeds -are approximated is the anterior. The extent to which the 
hinder part remains open varies with the position of the cheliped. 
The basis and iscJiiiiin are fused, though traces of their junction usually 
remain in the form of a groove. By this it is shown that the basis is a small, 
region which in width makes the transition from the stout coxa to the flat 
ischium, and in position fills a triangular gap between them, due to the fact 
that the ischium is displaced to the median side of the coxa. The free (median) 
edge of the basis abuts upon the edge of the sternum, and continues the 
contact made by the facet upon the coxa. On the outer side the basis bears 
