﻿Mr. 
  W. 
  L. 
  Distant 
  on 
  the 
  Genus 
  Plisthenes. 
  271 
  

  

  very 
  doubtful 
  at 
  present, 
  the 
  animal 
  being 
  unknown. 
  A 
  small 
  

   dried 
  portion 
  of 
  it 
  was 
  attached 
  to 
  the 
  shell, 
  but 
  no 
  odonto- 
  

   phore 
  could 
  be 
  discovered 
  in 
  it. 
  Some 
  minute, 
  glassy, 
  grain- 
  

   like 
  objects 
  adhering 
  to 
  it 
  proved 
  on 
  examination 
  under 
  the 
  

   microscope 
  to 
  be 
  minute 
  shells 
  and 
  doubtless 
  the 
  vouno- 
  Q 
  f 
  

   this 
  species 
  ; 
  there 
  is 
  therefore 
  every 
  reason 
  to 
  believe 
  that 
  it 
  

   is 
  viviparous. 
  

  

  The 
  genus 
  and 
  species 
  may 
  be 
  described 
  thus 
  : 
  — 
  

  

  K.OBILLARDIA. 
  

  

  Animal 
  parasiticum, 
  Echinos 
  incolens, 
  vivi- 
  

   parutn. 
  Testa 
  vitrea, 
  depresse 
  helici- 
  

   formis 
  ; 
  spira 
  brevis 
  ; 
  apertura 
  lunata, 
  

   magna 
  ; 
  peristoma 
  simplex, 
  tenue. 
  

  

  Robillardia 
  cernica 
  *. 
  

  

  \ 
  

  

  Testa 
  tenuis, 
  pellucida, 
  vitrea, 
  nitens, 
  im- 
  

   perforata, 
  depresse 
  globosa 
  ; 
  anfractus 
  5 
  

  

  celeriter 
  crescentes, 
  lirulis 
  suleisque 
  levibus 
  spiralibus 
  striisque 
  

   incrementi 
  arcuatis 
  sculpti, 
  convexi, 
  sutura 
  canaliculata 
  sejuncti 
  ; 
  

   ulfcimus 
  magnus, 
  ad 
  peripheriam 
  rotundatus, 
  inferne 
  concentrice 
  

   leviter 
  liratus 
  et 
  scrobiculatus, 
  in 
  regione 
  umbilici 
  plus 
  minus 
  

   tenuiter 
  callosus 
  ; 
  apertura 
  lunata 
  ; 
  peristoma 
  tenue, 
  margine 
  

   inferiore 
  recedente, 
  columellari 
  supra 
  leviter 
  expanso. 
  

  

  Diam. 
  maj. 
  8 
  mm., 
  min. 
  6| 
  ; 
  alt. 
  5. 
  

  

  Hah. 
  Mauritius. 
  

  

  This 
  delicate 
  shell 
  has 
  the 
  glassy 
  texture 
  of 
  Garinaria 
  and 
  

   somewhat 
  the 
  form 
  of 
  certain 
  species 
  of 
  Hyalinia. 
  

  

  XXVIII. 
  — 
  Synopsis 
  of 
  the 
  Rhynchotal 
  Genus 
  Plisthenes. 
  

   By 
  W. 
  L. 
  Distant. 
  

  

  This 
  genus 
  of 
  Tesseratominas, 
  founded 
  by 
  Stal 
  in 
  1864, 
  is 
  

   liable 
  by 
  the 
  large 
  size 
  and 
  general 
  resemblance 
  of 
  its 
  repre- 
  

   sentatives 
  with 
  their 
  almost 
  common 
  area 
  of 
  distribution 
  to 
  

   have 
  its 
  different 
  species 
  unrecognized. 
  This 
  was 
  absolutely 
  

   the 
  case 
  with 
  Stal, 
  who, 
  in 
  enumerating 
  the 
  genus 
  (En. 
  Hem. 
  

   i. 
  p. 
  63), 
  included 
  the 
  second 
  species, 
  P. 
  dilatatus, 
  Montr., 
  

   more 
  fully 
  redescribed 
  by 
  Vollenhoven, 
  as 
  a 
  synonym 
  of 
  P. 
  

   Meriance, 
  Fabr. 
  I 
  was 
  subsequently 
  (Trans. 
  Eut. 
  Soc. 
  1880, 
  

  

  • 
  From 
  " 
  Cerne," 
  an 
  old 
  name 
  of 
  the 
  island 
  of 
  Mauritius. 
  

  

  19* 
  

  

  