254 
Proceedings of the Koyal Society of Edinburgh. [Sess. 
The bearing surface is wholly confined to the lateral (or in the case of 
the posterior coxae to the posterior) aspect of the articular foramen. In 
this region alone does the foramen have any vertical depth. A smooth 
wall, arising dorsally from the border of each bay, sweeps from about the 
middle of eacli border of the Cupid’s heart round the bay to meet at the 
articular spur, the whole ventral surface of which is also smooth. These 
two hay-walls along with the ventral surface of the articular spur form 
the bearing surface. The articular spur, broad -based and heavily calcified, 
tapers to a point at the free end ; projecting ventrally near its base is a 
rounded boss — the coronoicl boss ; the apical part of the spur forms quite 
a pronounced hook, with its concavit}^, and therefore also its point, directed 
Fig. 3.— The left coxal foramen of the sixth true thoracic 
somite of Glyptonotus. x 2. 
a, coronoidboss ; b, posterior bay-wall ; c, terminal 
hook on articular spur ; d, anterior bay-wall. 
ventralwards. By means of this spur the weight of the body segment is 
transferred to the basipodite, which has a special knife-edge that rides 
across the concavity of the hook. This is therefore the most important 
part of the bearing surface, the smooth walls arising dorsally from the 
border of the two bays being merely guiding surfaces for corresponding 
rounded processes on the basipodite — see fig. 3. 
The whole lateral part of the coxa, which carries the bearing surface, 
and thus comes into direct contact with the basipodite, is evidently designed 
for strength. The dorsal wall, which fuses with the tergite, and the 
anterior and posterior (intersegmental) walls are likewise strong and rigid, 
offering in this respect a marked contrast to the ventral wall in its medial 
part, for the latter becomes progressively thinner and less rigid as we 
trace it towards the sternite. 
The articular part of the basipodite is in cross section much smaller 
than the articular foramen of the coxa. The bearing surface is limited 
