CYLIONUS. 127 
CYLIONUS*. 
Cylionus, Cook, Pr. U.S. Nat. Mus. xxi. p. 462 (1898). 
Ventral surface of the segments, external to the articulation of the legs, with the posterior border much 
straighter than in Spheriodesmus, only slightly sinuous and oblique in the anterior half of the body and 
becoming quite straight in its posterior half, with a small tooth and notch external to the posterior leg ; 
the anterior spiniform tubercle farther away from the base of the anterior leg. 
Body highly vaulted, distinctly compressed at the base of the keels, which, at least in the middle and posterior 
regions of the body, project slightly obliquely outwards and downwards. Most of the keels gradually 
narrowed and pointed; those of the fourth noticeably larger than of the fifth; those of the third 
at all events larger than in the genus Spheriodesmus. For sexual characters, see the description of 
C. constrictus and also of C. gracilis. 
Distribution. Mexico. 
The comparatively large size of the keels of the third segment and the small size of 
those of the fifth in this genus must be taken into consideration in judging of the 
possible relationships between Spheriodesmus and Cyclodesmus, the third segment 
being the largest of the series in the latter. It must also be remarked that the 
phallopods of C. constrictus show considerable similarity to these same organs in 
Cyclodesmus aztecus as depicted by Carl (Rev. Zool. Suisse, x. p. 678, t. 12. fig. 109, 
1902). 
The typical species of this genus, C. gracilis, Sauss., is unknown to me, except from 
the description that Saussure and Humbert have published. It was also unknown to 
Cook, who proposed to separate it generically from Spheriodesmus. I have therefore 
recharacterised Cylionus from the species described below as new, which in many 
of its characters unmistakably resembles C. gracilis. ‘The two may be distinct 
generically ; certainly they differ considerably in the structure of their copulatory 
organs. But in the present state of our knowledge it seems to me to be preferable to 
refer them to the same genus, rather than to erect another genus for this new species. 
By the structure of the phallopods the two known species of Cylionus may be 
distinguished as follows :— 
a. Phallopod terminating in two strongly incurved hooked processes, the upper 
bifid, the lower simple and pointed . . . 1. 1 1 ee ee eee ee gracilis. 
a‘. Phallopod terminally bent strongly upwards and outwards . . . . « . . « = constrictus. 
1. Cylionus constrictus, sp.n. (Tab. IX. figg. 5-5 f) 
Colour uniformly yellowish-white when cleared of adherent dirt. Body small and narrow, with the dorsal 
surface strongly convex and a shallow constriction at the base of the keels, which incline slightly out- 
wards. Antenne lightly incrassate; second, third, and sixth segments subequal and longer than the 
fourth and fifth. First tergal plate with postero-lateral border strongly arched, the lateral angle nearly 
rectangular. Keels of the second narrower and shorter than those of the third, which are comparatively 
large and extend inferiorly as low as those of the fourth, with anterior border convex and posterior border 
concave. Keels of fourth moderately large, much larger than those of the fifth, which are intermediate 
* Omitted from the Zool. Record, 1898. 
