﻿PROM 
  THE 
  WEALDEN 
  AND 
  PURBECK. 
  517 
  

  

  bones 
  of 
  the 
  pelvis 
  and 
  the 
  plastron. 
  This 
  specimen 
  differs 
  from 
  

   adult 
  examples 
  of 
  P. 
  Bullocki 
  not 
  only 
  in 
  the 
  above-mentioned 
  point, 
  

   but 
  also 
  in 
  the 
  relatively 
  wider 
  vertebral 
  and 
  costal 
  shields, 
  and 
  the 
  

   circumstance 
  that 
  the 
  first 
  marginal 
  bone 
  articulates 
  wholly 
  with 
  the 
  

   nuchal 
  instead 
  of 
  largely 
  with 
  the 
  first 
  costal 
  ; 
  while, 
  as 
  shown 
  by 
  

   other 
  specimens, 
  the 
  mesoplastral 
  terminates 
  outwardly 
  in 
  a 
  point. 
  

   All 
  these 
  features 
  must 
  apparently 
  be 
  regarded 
  as 
  characteristic 
  of 
  

   the 
  young. 
  

  

  The 
  second 
  specimen 
  (B.M. 
  No. 
  R. 
  1524) 
  is 
  a 
  portion 
  of 
  an 
  

   adult 
  plastron, 
  which 
  shows 
  both 
  the 
  dorsal 
  and 
  ventral 
  surfaces 
  of 
  

   the 
  bone. 
  It 
  agrees 
  in 
  all 
  respects 
  with 
  other 
  specimens 
  of 
  the 
  

   plastron 
  of 
  P. 
  Bullocki, 
  and 
  there 
  can 
  be 
  no 
  hesitation 
  in 
  referring 
  it 
  

   to 
  that 
  species. 
  

  

  Pig. 
  4. 
  — 
  Dorsal 
  Aspect 
  of 
  the 
  Posterior 
  Extremity 
  of 
  the 
  Plastron 
  of 
  

   Pleurosternum 
  Bullocki 
  ; 
  from 
  the 
  Purbeclc 
  of 
  Dorsetshire. 
  

   (J 
  nat. 
  size.) 
  

  

  hp.p., 
  hypoplastral 
  ; 
  cep., 
  xiphiplastral 
  ; 
  /., 
  facet 
  for 
  pubis. 
  

  

  The 
  peculiar 
  feature 
  of 
  this 
  specimen 
  (fig. 
  4) 
  is 
  the 
  presence 
  on 
  

   the 
  dorsal 
  aspect 
  of 
  the 
  xiphiplastral 
  of 
  a 
  facet 
  (/), 
  for 
  the 
  articu- 
  

   lation 
  of 
  the 
  pubis. 
  This 
  facet 
  occupies 
  precisely 
  the 
  same 
  position 
  

   as 
  in 
  the 
  plastron 
  of 
  the 
  Jurassic 
  Pleurodiran 
  genus 
  Plesiochelys, 
  

   where 
  only 
  the 
  pubis 
  unites 
  with 
  the 
  plastron 
  ; 
  and 
  also 
  corre- 
  

   sponds 
  to 
  the 
  pubic 
  articulation 
  in 
  existing 
  Pleurodirans, 
  where 
  

   both 
  pubis 
  and 
  ischium 
  unite 
  with 
  the 
  xiphiplastral. 
  Whereas, 
  

   however, 
  in 
  true 
  Pleurodira, 
  the 
  union 
  between 
  the 
  pelvis 
  and 
  

   plastron 
  is 
  a 
  sutural 
  one, 
  in 
  the 
  present 
  instance 
  these 
  bones 
  appear 
  

   merely 
  to 
  have 
  articulated 
  by 
  smooth 
  facets. 
  

  

  If 
  I 
  am 
  right 
  in 
  referring 
  these 
  . 
  two 
  types 
  of 
  pelvis 
  to 
  a 
  single 
  

   species 
  (and 
  in 
  any 
  case 
  they 
  indicate 
  extremely 
  nearly 
  allied 
  forms 
  

   which 
  cannot 
  be 
  generically 
  separated), 
  it 
  would 
  appear 
  that 
  while 
  

   the 
  young 
  of 
  Pleurosternum 
  had 
  a 
  type 
  of 
  pelvic 
  structure 
  similar 
  

   to 
  that 
  which 
  obtains 
  in 
  the 
  Cryptodira, 
  the 
  adult 
  approximated 
  to 
  

   the 
  Pleurodiran 
  modification. 
  This 
  being 
  so, 
  it 
  remains 
  to 
  consider 
  

   whether 
  we 
  are 
  to 
  regard 
  these 
  Chelonians 
  as 
  Cryptodirans 
  approxi- 
  

   mating 
  to 
  the 
  Pleurodira, 
  or 
  as 
  very 
  generalized 
  Pleurodirans, 
  or 
  as 
  

   the 
  representatives 
  of 
  a 
  section 
  distinct 
  from 
  both. 
  

  

  This, 
  I 
  admit, 
  is 
  a 
  question 
  of 
  some 
  difficulty 
  ; 
  but 
  since 
  it 
  is 
  practi- 
  

   cally 
  certain, 
  as 
  M. 
  Dollo 
  has 
  pointed 
  out, 
  that 
  we 
  must 
  regard 
  both 
  

   the 
  Cryptodira 
  and 
  Pleurodira 
  as 
  divergent 
  branches 
  from 
  an 
  original 
  

  

  