﻿518 
  CHELONIAN 
  REMAINS 
  FROM 
  THE 
  WEALDEN 
  AND 
  PURBECX. 
  

  

  common 
  stock, 
  it 
  is 
  quite 
  evident 
  that 
  such 
  stock 
  must 
  have 
  had 
  a 
  

   plastron 
  of 
  very 
  much 
  the 
  type 
  of 
  that 
  of 
  the 
  Pleurosternidae 
  ; 
  that 
  is 
  to 
  

   say 
  there 
  must 
  have 
  been 
  a 
  mesoplastral 
  bone 
  and 
  an 
  intergular 
  shield, 
  

   since 
  these 
  features, 
  if 
  once 
  lost, 
  would 
  be 
  very 
  unlikely 
  to 
  reappear. 
  

   Further, 
  we 
  should 
  expect 
  such 
  en 
  ancestral 
  type 
  to 
  show 
  such 
  dif- 
  

   ferences 
  in 
  the 
  relation 
  of 
  the 
  pelvis 
  to 
  the 
  plastron 
  as 
  we 
  find 
  

   obtaining 
  in 
  the 
  specimens 
  before 
  us. 
  If 
  we 
  refer 
  the 
  Plenrosternidae 
  

   to 
  the 
  Cryptodira, 
  we 
  should 
  destroy 
  the 
  definition 
  of 
  that 
  section 
  by 
  

   the 
  inclusion 
  of 
  a 
  form 
  with 
  a 
  union 
  between 
  the 
  pelvis 
  and 
  the 
  

   plastron 
  ; 
  while 
  if 
  we 
  assign 
  them 
  to 
  the 
  Pleurodira, 
  we 
  should 
  

   equally 
  invalidate 
  the 
  definition 
  of 
  that 
  group, 
  since 
  we 
  should 
  have 
  

   to 
  include 
  a 
  genus 
  with 
  a 
  free 
  pelvis 
  in 
  the 
  young. 
  

  

  Under 
  these 
  circumstances, 
  it 
  appears 
  to 
  be 
  the 
  preferable 
  course 
  

   to 
  regard 
  this 
  family 
  as 
  the 
  representative 
  of 
  a 
  generalized 
  section, 
  

   of 
  which 
  the 
  earlier 
  (unknown) 
  members 
  were 
  the 
  common 
  ancestors 
  

   of 
  the 
  Cryptodira 
  and 
  Pleurodira 
  ; 
  and 
  I 
  accordingly 
  propose 
  for 
  this 
  

   section 
  the 
  name 
  of 
  Amphichelydia. 
  

  

  The 
  Neocomian 
  genus 
  Helochelys 
  will 
  certainly 
  come 
  in 
  the 
  Pleuro- 
  

   sternidae 
  ; 
  while 
  I 
  think 
  the 
  Baenidae 
  of 
  Prof. 
  Cope, 
  as 
  represented 
  

   by 
  the 
  Upper 
  Jurassic 
  Platychelys 
  and 
  the 
  Eocene 
  Baena, 
  may 
  pro- 
  

   bably 
  be 
  likewise 
  included 
  in 
  the 
  same 
  family, 
  and 
  will 
  certainly 
  

   come 
  in 
  the 
  same 
  section. 
  Prof. 
  Cope 
  has, 
  indeed, 
  remarked 
  on 
  the 
  

   peculiarly 
  generalized 
  affinities 
  of 
  Baena, 
  which 
  he 
  regards 
  as 
  exhi- 
  

   biting 
  decided 
  evidence 
  of 
  affinity 
  with 
  the 
  Pleurodira, 
  especially 
  in 
  

   the 
  approximation 
  towards 
  a 
  union 
  between 
  the 
  pelvis 
  and 
  the 
  

   plastron. 
  

  

  The 
  Amphichelydia, 
  as 
  thus 
  exemplified, 
  will 
  include 
  all 
  those 
  

   forms 
  hitherto 
  referred 
  to 
  the 
  Cryptodira 
  which 
  possess 
  a 
  mesoplastral 
  

   bone, 
  and 
  will 
  thus 
  enable 
  us 
  to 
  add 
  to 
  the 
  definition 
  of 
  that 
  section 
  

   the 
  absence 
  of 
  this 
  bone. 
  

  

  Finally, 
  I 
  may 
  observe 
  that 
  the 
  pectoral 
  girdle 
  and 
  humerus 
  of 
  

   Fleurosternum 
  are 
  of 
  a 
  decidedly 
  Pleurodiran 
  type, 
  coming 
  near 
  to 
  

   those 
  of 
  the 
  existing 
  Chelys. 
  I 
  have, 
  indeed, 
  studiously 
  avoided 
  all 
  

   reference 
  to 
  the 
  structure 
  of 
  the 
  skull 
  and 
  neck, 
  which 
  affords 
  such 
  

   an 
  important 
  distinction 
  between 
  the 
  existing 
  members 
  of 
  the 
  Crypto- 
  

   dira 
  and 
  Pleurodira, 
  since 
  it 
  will 
  be 
  quite 
  evident 
  that 
  any 
  evidence 
  

   adduced 
  from 
  them 
  can 
  have 
  no 
  possible 
  bearing 
  in 
  a 
  case 
  where 
  

   their 
  structure 
  is 
  totally 
  unknown. 
  

  

  Discussion. 
  

  

  The 
  Chairman 
  said 
  that 
  some 
  interesting 
  points 
  of 
  difference 
  

   between 
  the 
  living 
  and 
  extinct 
  forms 
  of 
  Chelonia 
  had 
  been 
  well 
  

   brought 
  out 
  by 
  the 
  Author. 
  

  

  Prof. 
  Blake 
  inquired 
  in 
  what 
  state 
  the 
  horny 
  scutes 
  of 
  the 
  

   Chelonia 
  are 
  preserved. 
  

  

  The 
  Author 
  said 
  by 
  impressions 
  on 
  the 
  underlying 
  bone. 
  In 
  the 
  

   Stonesfield 
  Slate 
  the 
  scutes 
  themselves 
  of 
  the 
  Testudo 
  StricMandi 
  of 
  

   Phillips 
  are 
  preserved. 
  

  

  