343 



This species may be at once recognised from the only other known 

 member of the genus by its much smaller size (6 mm. as compared 

 with 10), shorter legs and antennae, differently shaped keels, and by 

 slight sexual characters, for instance the femur of the 5 th leg in front 

 of the copulatory organ is much more strongly inflated than in H. doriae , 

 as also is this segment in the anterior half of the apparatus. 



Sub-order polydesmoidea , Pocock *). 

 Family POLYDESMIDAE. 



Platyrhachus , C. Koch. 



Since the genus Stenonia was characterised without a single spe- 

 cies being referred to it as a type, it seems to me that it had better 

 be discarded. I consequently adopt C. Koch's generic name for all 

 the species that have been hitherto described as Stenonia. I also 

 include under this heading Odontodesmus and Acanthodesmus. 



Synopsis of the species of Platyrhachus. 



a. The keels distinctly bidentate , sometimes with minor denticulations ; 

 body slender, with the legs and antennae longer; tail squared. 



a 1 . Dorsal surface smoother: the two teeth of the keels subequal, 

 without minor denticulations; pores situated at the base of the 

 anterior tooth of the keel; keel not basally shouldered. 

 a 2 . Keels smaller , with only the tips of the teeth flavous . . bidens. 

 b 2 . Keels larger, and almost entirely flavous aequidens. 



b l . Dorsal surface rather coarsely granular, the posterior tooth of 

 the keels the largest, either of the teeth armed with a small 

 denticle ; pore situated opposite the middle of the excision ; keel 

 distinctly shouldered inaequidens. 



b. The keels at the anterior end of the body at least with entire 

 margins. 



a 3 . The keels from the 5 th backwards with lateral margins cut 

 out into two strong, sometimes bifid, teeth, between which 

 there is often a smaller tooth (keels not shouldered, pore close 

 to the edge of the emargination) , . . . weberi. 



b 3 . The keels from the 5 th backwards, not bilobate. 



1) It appears to me to be probable that this sub-order will ultimately prove to be 

 divisible into several groups of the value of families. 



