﻿516 
  MB. 
  E. 
  LYDEXKEE 
  ON 
  CEETAIN 
  CHELONCAN 
  EEMAINS 
  

  

  figure 
  shows 
  that 
  there 
  is 
  some 
  confusion 
  between 
  bony 
  sutures 
  

   and 
  the 
  sulci 
  formed 
  by 
  horny 
  shields, 
  so 
  that 
  it 
  is 
  quite 
  possible 
  the 
  

   figure 
  may 
  be 
  incorrect 
  in 
  this 
  respect. 
  And 
  even 
  if 
  correct, 
  I 
  should 
  

   be 
  disposed 
  to 
  regard 
  this 
  feature 
  merely 
  as 
  an 
  individual 
  abnor- 
  

   mality, 
  seeing 
  that 
  the 
  associated 
  plastron 
  presents 
  no 
  characters 
  by 
  

   which 
  it 
  can 
  be 
  specifically 
  distinguished 
  from 
  that 
  of 
  P. 
  Bullocki, 
  

  

  Pig. 
  3. 
  — 
  The 
  imperfect 
  Shell 
  of 
  an 
  im.mature 
  individual 
  of 
  Pleuro- 
  

   sternum 
  Bullocki 
  ; 
  from 
  the 
  Purbeck 
  of 
  Swanage. 
  (f 
  nat. 
  size.) 
  

   (B. 
  M. 
  INo. 
  48262); 
  

  

  The 
  costals 
  of 
  the 
  right 
  side 
  have 
  been 
  removed 
  in 
  order 
  to 
  exhibit 
  

   the 
  dorsal 
  aspect 
  of 
  the 
  plastron. 
  

   m. 
  1, 
  first 
  marginal 
  bones 
  ; 
  mi., 
  nuchal 
  ; 
  n. 
  \-n. 
  7, 
  neurals 
  ; 
  c. 
  1-c. 
  8, 
  costals 
  ; 
  

   ep., 
  epiplastral 
  ; 
  ent.p., 
  entoplastral 
  ; 
  hy.p., 
  hyoplastral 
  ; 
  m.p., 
  meso- 
  

   plastral 
  ; 
  hp.p., 
  hypoplastral 
  ; 
  x.p., 
  xiphiplastral. 
  

  

  and 
  bearing 
  in 
  mind 
  that 
  if 
  this 
  feature 
  be 
  regarded 
  as 
  specific, 
  we 
  

   should 
  have 
  to 
  refer 
  all 
  the 
  young 
  specimens 
  to 
  P. 
  concinnum, 
  and 
  all 
  

   the 
  adult 
  specimens 
  (except 
  the 
  solitary 
  type 
  of 
  the 
  latter) 
  to 
  P. 
  Bul- 
  

   locki. 
  I 
  accordingly 
  regard 
  the 
  specimen 
  represented 
  in 
  fig. 
  3 
  as 
  

   probably 
  belonging 
  to 
  a 
  young 
  individual 
  of 
  the 
  latter 
  and 
  only 
  

   definable 
  Purbeck 
  species 
  of 
  the 
  genus. 
  The 
  importance 
  of 
  this 
  

   specimen 
  is 
  that 
  it 
  shows 
  the 
  absence 
  of 
  any 
  connexion 
  between 
  the 
  

  

  