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  5th  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  1). 
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. 
a1.  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. 
a2.  Keels  smaller ,  with  only  the  tips  of  the  teeth  flavous .  .  bidens. 
b2.  Keels  larger,  and  almost  entirely  flavous  aequidens. 
bl.  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. 
a3.  The  keels  from  the  5th  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. 
b3.  The  keels  from  the  5th  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. 
