120 DIPLOPODA. 
differences, nevertheless, are extremely difficult to express in writing, since they are 
mostly differences of degree. The males, on the contrary, present admirable secondary 
sexual characters in the structure of the legs of the first pair and of the phallopods, 
and in the relative width of the sternal areas adjacent to the socket which carries 
these organs. Fortunately, judging from the material of the genus that has been 
examined, males are more abundant than females, and in every species that has been 
described the principal secondary sexual characters of the males are known. 
The following table, based upon characters other than those appertaining to sex, 
may perhaps be of some service in determining the forms that I have been able to 
examine; but I have been unable to include in it species of which I have not seen 
examples :— 
a. Dorsal surface of segments comparatively coarsely rugose; fourth tergal plate 
somewhat strongly convex and abruptly raised behind its anterior rim . . . coriaceus. 
a’. Dorsal surface smooth or only finely rugulose ; fourth tergal plate much flatter, 
lightly convex antero-posteriorly. 
b. Posterior edge of the eighteenth segment forming an evenly rounded curve from the 
summit to the posterior angle of the keel—that is to say, the posterior border 
of the keel and of the dorsal portion meeting at a widely rounded angle. 
c Keels of the fourth segment forming a comparatively strong sigmoid flexure ; 
keels of segments 6 to 9 comparatively broad . . . . . . . . | digitatus. 
. Keels of the fourth segment with a much less pronounced sigmoid flexure ; 
keels of segments 6 to 9 narrower. 
d. Keels of the fourth narrower, the anterior angle less strongly rounded . oniscus. 
da’. Keels of the fourth broader, with the anterior angle more strongly rounded. 
e. Colour olive-green: larger—length 27 mm., width 10 mm. . . . . robustus. 
e'. Colour sandy-white: smaller—length 15 mm., width5 mm. . . . . = prehensor. 
6’. Posterior edge of the dorsal portion of the eighteenth segment forming a 
distinct angle with the posterior border of the keel. 
f. Posterior border of the keels lightly concave. . . 2°... Stilifer. 
f*. Posterior border of the keels straight . . . . 2... 2... ee) angustus. 
By the secondary sexual characters of the males the species enumerated below may 
be distinguished as follows :— 
a, Tibio-femoral segment of phallopod markedly biramous—that is to say, with a 
long, stout, or slender, often thumb-like subsidiary branch, which is not very 
much shorter than the principal branch. 
b. Phallopod very long and slender and comparatively straight ; the two branches 
also long and slender, the main branch sinuously curved and undivided at the 
apex ; the subsidiary branch projecting forwards, lightly curved at the base ; 
femur of first leg with strong basal tooth ; sternal areas of fifth, sixth, and 
seventh segments, and socket of phallopods, narrow. . . . - . « « digitatus. 
b', Phallopod short and stout ; sternal areas of fifth, sixth and seventh segments, 
and socket of phallopods, broader. 
