RHYSODESMUS. 207 
According to the describers, this species is distinguishable from all the Mexican 
forms known at that time by the subspiniform posterior angles of the keels and the 
emargination of their posterior borders. 
22. Rhysodesmus otomitus. 
Polydesmus (Fontaria) otomitus, Sauss. Linn. Ent. xiii. p. 322 (1859)'; Mém. Soc. Phys. Genéve, 
xv. p. 315, t. 2. fig. 12 (1860) *; Sauss. & Humb. Miss. Sci. Mex., Myr. p. 37 (1872) *. 
Q. Colour? ; whitish when dry. Very like the male of R. fraternus, but relatively a little less convex. 
Posterior border of keels scarcely concave as far back as the 16th, marginal thickenings which bear 
the pores thickened and lozenge-shaped. Pores large, in the middle of the lateral border, even at the 
posterior extremity of the body. 
3. Differs from the 2 by having the posterior angle of the keels more produced and the keels more 
elongated. 
Length, 9, 24 millim., width 6. 
Hab. Mexico, the plateau and temperate regions (Cordova) ! 2, 
This species was based upon immature individuals. The measurements given 
therefore are deceptive for the adult. It is said to differ from R. fraternus by having 
the pores lodged in small pits, in being less convex, and in having the keels of the 
male otherwise elevated. ‘To what extent these characters would apply to adults is 
unknown. 
23. Rhysodesmus simillimus. 
Polydesmus (Fontaria) simillimus, Humb. & Sauss. Rev. et Mag. Zool. (2) xxi. p. 150 (1869) '; 
Miss. Sci. Mex., Myr. p. 31, t. 1. figg. 5-5 6 (1872) °. 
g. Colour wholly pale when dry. Body tolerably flat. Acels continuous when the body is contracted. 
Dorsal surface smooth but coriaceous. eels scarcely elevated, with narrow marginal thickening and 
very small pores; behind the 7 th segment the keels are directed slightly obliquely backwards, with the 
anterior angle rounded, the posterior border lightly concave, with the posterior angle sharp; from the 
Ath to the 16th segment a small tooth-like “‘shoulder” process is seen at the base of the posterior 
border of each keel; the keels from the 15th to the 19th are large and triangularly prolonged. 
Posterior extremity of the body much narrowed. 
Length of ¢ (contracted) 25 millim., width 5:4. 
Hab. Mexico, Eastern Cordillera (Santa Cruz, near Orizaba) !?. 
Said to resemble closely 2. consobrinus, otomitus, and zapotecus, but to differ from 
the first two in the presence of the small tooth upon the base of the posterior border 
of the keels, and from R. zapotecus in having a distinct notch at the base of the keels of 
the 17th and 18th segments, and the keels of the 19th larger and more triangular. 
24. Rhysodesmus tepanecus. 
Pelydesmus (Fontaria) tepanecus, Sauss. Linn. Ent. xiii, p. 8321 (1859) *; Mém. Soc. Phys. Genéve, 
xv. p. 319, t. 3. fig. 17 (1860) °; Humb. & Sauss. Miss. Sci. Mex., Myr. p. 30, t. 1. figg. 4-46 
(1872) °. 
Colour ; a broad median dark brown band extending along the metazonites, the patch on each segment 
narrowly bordered in front and behind by yellow and not extending laterally on to the keels, which are 
