RHYSODESMUS. 191 
b. Sterna from 5th to 17th segments posteriorly markedly bidentate; pores 
far forwards; body robust; keels depressed, their posterior angles 
produced; prozonites pale, metazonites dark ; interzonal groove as under 
section B . . we ee ee ee ew ee tabascensis. 
b*. Sterna not remarkably bidentate, the posterior border straight, lightly 
convex or lightly concave. 
c. Ridge on anterior border of keels continued across the dorsum as a crest, 
forming the anterior rim of the interzonal groove, this groove being 
thus separated from the corresponding groove on the lateral surface of 
the segments; lateral portion of first tergal plate narrow and acuminate, 
dorsal surface granular, at least on the keels; body very or moderately 
broad ; keels depressed with produced posterior angles; sterna wide 
anteriorly ; pores anterior; prozonites pale; metazonites dark. 
d. Body broad, about 12 mm. when the length is 47 mm.; posterior angle 
of keels very slightly produced ; anal sternal plate almost semi- 
circular, with the setiferous tubercles widely separated ; phallopod with 
principal branch slightly shorter than palmar portion of the segment, 
inclined inwards distally and upcurled at the point ; auxiliary branch 
long, curved, somewhat blade-like, directed obliquely upwards basally 
and about half as long as the palmar portion. . . . . . . « . stolld. 
d'. Narrower, about 9 mm. with a length of 44 mm.; posterior angle of 
keels more markedly produced; anal sternal plate triangular, the 
setiferous tubercles more narrowly spaced. . . . . «© «© « « «+ champion. 
c'. Ridge on anterior border of keels not continued across the dorsum and 
separated from the anterior border of the groove between the prozonite 
and metazonite which is continued entirely round the segment ; lateral 
portion of first tergal plate broader, not markedly acuminate; upper 
surface of keels rarely closely granular, though often with scattered 
granules or small tubercles arranged in three rows; when closely 
granular or coriaceous, the posterior angle is convex (e. gy. in 
_R. totanacus). 
e. Body nearly flat; keels high, wide, nearly horizontal ; form not compact, 
but “ Leptodesmus”’-like; keels of second segment wider than median 
dorsal area of metazonite; anterior border of keels extended well 
beyond the convexity of the ridge when this is marked; antenne 
and legs long, tarsi long and slender; sterna not wider in front than 
behind ; prozonites and metazonites uniformly brown; keels yellow. 
f. Larger, length about 48 mm. ; anterior crest of keels with marked 
convex curve near base; phallopod with its principal branch a little 
longer than the palmar portion, its distal smooth extremity very 
broad at base, abruptly narrowed and curved inwards, then forwards 
and a little outwards, the extremity is thus hook-like and externally 
geniculate, but the apex is not upeurled; the auxiliary branch very 
long, a little shorter than the palmar portion and directed forwards 
nearly parallel with the main branch . . we ee ee + + godmani. 
