RHACHIDOMORPHA. ‘175 
name wneinata. This author showed that the phallopod of R. adunca has no coxal 
calcar, and thus differs from that organ as known in all the species modern authors 
have referred to Leptodesmus. There is no evidence, however, that Attems knew 
the male of the species he identified as R. adunca. ‘The male was known to him in 
the case of the species he determined as R. tarasca. But all that I know of the 
distribution of species of Diplopods justifies the conclusion that the two species 
from Espirito Santo must be specifically different from the genuine R. adunca and 
ft. tarasca from Mexico. If this be so, as is practically certain, they may also be 
generically different. At all events, it is proved that 2. adunca does not belong to 
the genus Leptodesmus, and I do not think the evidence justifies the conclusion that 
ft. tarasca is generically distinct from 2. adunca. I have tentatively, therefore, kept 
them under the same generic heading, Rhachidomorpha, of which the type-species is 
ft. tarasca, with Microrhachis, of which the type-species is R. adunca, as its synonym. 
Examination of the male-characters of 2. tarasca alone can show whether this opinion 
is correct or whether &. ¢arasca, carrying with it the generic name Rhachidomorpha, 
belongs to the same category of species as Leptodesmus and differs generically from 
ft. adunca. If this be so, the genus Microrhachis will have to be resuscitated in the 
section Rhachodesmine. 
The two known species may be distinguished as follows :— 
a. Keels very strongly elevated and corniform. . . . .. . . . . . . . .~ tarasca. 
6. Keels only moderately elevated. 2. 2. 2. 1 ww ee eee ee dunce. 
1. Rhachidomorpha tarasca. 
Polydesmus (Rhachidomorpha) tarasca, Sauss. Mém. Soc. Phys. Geneve, xv. p. 327, t. 4. fig. 24 
(1860). 
3. Colour? Body slender, elongate, and smooth. Antenne very long and slender. First tergal plate with 
its anterior border semicircularly arched; its keels strongly aliform, elevated, with posterior border 
concave, ending in a sharp backwardly-directed spine. Acels of 2nd and 3rd segments directed slightly 
forwards at the base, then strongly recurved ; those of the following segments in the form of narrow 
wings, strongly elevated, terminated by a sharp backwardly-directed spine ; marginal thickening narrow ; 
the pores not far from the extremity of their spiniform processes; a small spiniform tooth near the 
anterior end of the outer side of the keels. The keel-bearing portion of all the segments with an arched 
groove extending across from the base of one keel to that of the other. Caudal process of anal tergal 
plate conical. 
Length 21 millim., width ? 
Hab. Mexico, Cordova}, 
2. Rhachidomorpha adunca. 
Polydesmus (Rachidomorpha) uncinatus, Humb. & Sauss. Rev. et Mag. Zool. (2) xxi. p. 152 (1869) ' ; 
Miss. Sci. Mex., Myr. p. 38, t. 1. fig. 14 (1872) * (wncinatus preoccupied). 
Polydesmus (Rachidomorpha) aduncus, Sauss. & Humb, Miss. Sci. Mex., Myr. p. 158 (1872)’*, 
