206 DIPLOPODA. 
The keels are very large but distinctly depressed, almost following the slope of the 
dorsum, with the posterior angles dentiform. ‘The dorsal surface is rugose, tubercular 
laterally. The sterna are deeply emarginate, being bilobate or bidentate posteriorly ; 
they are also much wider in front than behind. ‘The terminal segments of the legs 
are quite short. The phallopods are elongate and distally gradually incurved; seen 
from the side the principal branch projects straight forwards and curls upwards 
at the tip and is about as long as the palmar area of the organ; the inter-ramal 
space is oval; the auxiliary branch is short, not half the length of the palmar 
portion, and is fairly evenly curved and subspiniform. This example is labelled 
‘‘ Mexico,” without further particulars as to locality. 
20. Rhysodesmus mystecus. 
Polydesmus (Fontaria) mystecus, Humb. & Sauss. Rev. et Mag. Zool. (2) xxi. p. 150 (1869) '; 
Miss. Sci. Mex., Myr. p. 82, t. 2. figg. 3-3 ¢ (1872) °. 
2. Colour (in alcohol) testaceous with a pair of brownish-red longitudinal dorsal bands extending laterally 
near the base of the keels over the metazonites and prozonites; first tergal plate with a somewhat 
A-shaped mark; head blackish above, testaceous below; antennz and legs testaceous, last segment of 
the former brown. 
Very like R. acolhuus, a little more vaulted, the keels less rounded, squarer, the posterior angles rectangular, 
and the marginal thickening narrower, the pores opening almost laterally. The body much less attenuated 
posteriorly, remaining wide to the end, with the keels of the three penultimate segments much less 
prolonged ; those of the 19th forming only small triangular teeth, whereas in 2. acolhuus they form 
longer and wider lobes. The caudal process is, moreover, very different, being convex, conical, wider 
than long, and more pointed, with concave lateral borders. 
3. Much less vaulted, with the keels horizontal, wide and short, showing an oblique backward inclination 
from the 7th segment, the posterior end narrower. 
Length, 2, 43 millim., width 9. 
29 3; 36 ” 9 8°3, 
Hab. Mexico, mid-region of the Eastern Cordillera ! 2. 
21. Rhysodesmus nahuus. 
Polydesmus (Fontaria) nahuus, Humb. & Sauss. Rev. et Mag. Zool. (2) xxi. p. 150 (1869)'; Miss. 
Sci. Mex., Myr. p. 36, t. 1. figg. 6-6 6 (1872) ®. 
Colour (in alcohol) greyish, the prozonites reddish-brown; a brown spot on each side of the metazonites 
placed anteriorly and at the base of the keels. Dorsal surface coriaceous, with some granules upon the 
keels. Body anteriorly attenuated. eels continuous and depressed, following the slope of the back ; 
those of segments 2-4 transverse with parallel borders; those of the 5th with a small basal prominence 
beLind, the following with a similar prominence, but with the posterior border concave; from the 7th 
or 8th segment the concavity becomes more and more pronounced, making the posterior angle produced 
and subspiniform, the anterior angle rounded; the subspiniform process disappearing about the 15th or 
16th segment. Acels of the 18th triangular, of the 19th small, rounded. Pores small, on the upper side 
of the middle of the marginal thickenings. Caudal process conical. 
g. Keels lightly raised posteriorly. . 
Length 30 millim., width 6. 
Hab. Mexico. Eastern Cordillera ! 2, 
