174 DIPLOPODA. 
The descriptions of this species furnished by Saussure and Carl compel me to regard it as distinct from the 
form described above as S. geddest. According to Carl’s figures of the phallopod, the apical or distal 
hollow in S. cyaneus is much longer and narrower, has no spiniform teeth on its proximal margin, and 
the seminal stile instead of being semicircularly curved and directed straight inwards, is sharply 
geniculate at the base and projects decidedly downwards and inwards with a sinuous curvature. 
In general form the two species are much alike, but de Saussure represents the anterior borders of 
the keels as considerably more convex than they are in S. geddesi, especially on the first tergal plate 
are the anterior borders of the keels produced and convex, thus giving rise to the median concavity he 
describes and figures. This does not exist in S. geddesi, where the anterior border forms from side to 
side a continuous curve, strongly pronounced laterally. Lastly, he makes no mention of any denticle 
on the antero-lateral border of the keels nor of a tooth upon the posterior angle, the margin of the keels 
forming, according to his figure, a continuous curve. Perhaps no great reliance should be placed upon 
the differences in colour; it must be borne in mind, however, that he describes S. cyaneus as green and 
represents the legs and keels as the same colour as the body. His animal is also larger, the length 
being 47 millim. and the width 7. 
Hab. Mexico, Orizaba !~6, 
RHACHIDOMORPHA. 
Rhachidomorpha, Saussure, Mém. Soc. Phys. Genéve, xv. p. 326 (1860); Saussure & Humbert, 
Miss. Sci. Mex., Myr. p. 37 (1872). 
Rhachidomorpha, Attems, Denk. Akad. Wien, lxvii. p. 410 (1899). 
Microrhachis, Carl, Rev. Suisse Zool. xi. p. 556 (1903). 
Distinguishable from all the Central-American genera by the shape of the keels, which are well developed, 
separated, high on the sides, elongate, spiniform, and tilted upwards so that the dorsal surface is flat or 
hollow, the degree to which the keels are tilted depending upon the sex and species, the tilting being 
greater in the male than in the female and greater in the typical species tarasca than in adunca. Pores 
normal, Phallopods, where known, much like those of Rhachodesmus ; the basal segment without calcar ; 
the proximal end of the distal segment with a roundish seminal fossa, the distal end with a seminal 
stile and a large bifid subsidiary branch. 
Type, R. tarasca, Sauss. 
Distribution. Mexico. 
I have added Microrhachis to the synonymy of Rhachidomorpha, because I cannot 
find any evidence that satisfies me as to the existence of generic characters to distin- 
cuish the typical species of the two, namely tarasca and adunca. Microrhachis was 
based upon the male-characters of the latter; but since the male of the genuine 
tarasca does not appear to have been examined for the particular points presented by 
adunca, there is very little evidence, much less procf, of their generic distinctness. 
On the other hand, the two species are so much alike in general features, especially in 
the unusual shape and direction of the keels, that strong presumptive evidence is 
supplied of the resemblance extending to deeper-seated structures. Attems, however 
(Mt. Mus. Hamburg, xviii. pp. 85, 95, 1901), records these two species, namely 
kk. tarasca and FR. adunca, from Espirito Santo in Brazil; and, as the result of his 
examination of the specimens so identified, reduced Lhachidomorpha to a subgeneric 
synonym of Leptodesmus. ‘This conclusion is opposed to that of Carl, who had the 
opportunity of examining the type of #. adunca, for which he wrongly retained the 
