﻿

  PROM 
  THE 
  WEALBEN 
  AND 
  TURBECK. 
  513 
  

  

  one 
  may 
  be 
  named 
  interpectoral. 
  On 
  this 
  view 
  the 
  gnlars 
  will 
  

   have 
  been 
  placed 
  anteriorly 
  to 
  the 
  intergular 
  in 
  the 
  same 
  manner 
  

   as 
  in 
  Chelodina. 
  

  

  Our 
  specimens 
  indicate, 
  therefore, 
  a 
  Chelonian 
  of 
  medium 
  size, 
  

   characterized 
  by 
  the 
  occurrence 
  of 
  a 
  row 
  of 
  median, 
  and 
  probably 
  

   azygos, 
  plastral 
  shields, 
  dividing 
  the 
  normal 
  plastral 
  shields 
  below 
  

   the 
  gulars. 
  This 
  series 
  is 
  a 
  continuation 
  of 
  the 
  intergular 
  now 
  

   found 
  in 
  all 
  Pleurodira 
  and 
  some 
  Cryptodira, 
  and 
  may 
  in 
  all 
  proba- 
  

   bility 
  be 
  regarded 
  as 
  indicating 
  an 
  archaic 
  type 
  of 
  structure, 
  the 
  

   Chelonian 
  plastron 
  having 
  probably 
  been 
  developed 
  from 
  abdominal 
  

   ribs 
  like 
  those 
  of 
  SpJienodon, 
  and 
  apparently 
  showing 
  a 
  tendency 
  to 
  

   the 
  obliteration 
  of 
  some 
  of 
  its 
  elements 
  with 
  advancing 
  specializa- 
  

   tion. 
  No 
  existing 
  Chelonian 
  exhibits 
  this 
  multiplication 
  of 
  plastral 
  

   shields 
  ; 
  but 
  Mr. 
  Boulenger 
  has 
  figured 
  a 
  minute 
  interanai 
  shield 
  

   in 
  two 
  specimens 
  of 
  Macroclemmys, 
  one 
  of 
  which 
  also 
  exhibits 
  an 
  

   equally 
  minute 
  azygos 
  shield 
  in 
  the 
  centre 
  of 
  the 
  plastron. 
  

  

  A 
  feature 
  of 
  a 
  somewhat 
  analogous 
  nature 
  to 
  that 
  characterizing 
  

   the 
  plastron 
  under 
  consideration 
  is, 
  however, 
  found 
  in 
  the 
  carapace 
  

   of 
  a 
  Chelonian 
  from 
  the 
  Kimeridgian 
  of 
  Hanover, 
  figured 
  by 
  Dr. 
  

   Portis 
  in 
  the 
  ' 
  Palseontographica,' 
  vol. 
  xxv. 
  pi. 
  xv., 
  under 
  the 
  name 
  

   of 
  Trojpidemys 
  Seebachi. 
  In 
  that 
  specimen 
  the 
  normal 
  azygous 
  series 
  

   of 
  vertebral 
  shields 
  is 
  divided 
  into 
  two 
  lateral 
  series 
  by 
  a 
  more 
  

   numerous 
  row 
  of 
  small 
  intervertebral 
  shields, 
  nearly 
  corresponding 
  

   in 
  number 
  with 
  the 
  underlying 
  neural 
  bones. 
  In 
  the 
  characters 
  of 
  

   the 
  bony 
  elements 
  of 
  the 
  carapace 
  that 
  specimen 
  corresponds 
  closely 
  

   with 
  typical 
  species 
  of 
  Tropideinys 
  • 
  but 
  the 
  multiplication 
  of 
  the 
  

   shields 
  should 
  not 
  improbably 
  be 
  regarded 
  as 
  a 
  generic 
  character. 
  

   It 
  occurs 
  to 
  me 
  that 
  the 
  carapace 
  of 
  the 
  Wealden 
  form 
  may 
  perhaps 
  

   have 
  had 
  a 
  similar 
  series 
  of 
  intervertebral 
  shields. 
  

  

  b. 
  The 
  Affinities 
  of 
  Pleurosternum. 
  

  

  The 
  Purbeck 
  Chelonian 
  to 
  which. 
  Sir 
  R. 
  Owen 
  applied 
  the 
  name 
  

   Pleurosternum 
  latiscutatum, 
  as 
  has 
  been 
  shown 
  by 
  Mr. 
  Poulenger 
  and 
  

   myself, 
  has 
  no 
  connexion 
  with 
  the 
  Pleurosternidse 
  *, 
  but 
  belongs 
  to 
  

   the 
  Plesiochelyidse 
  ; 
  and 
  from 
  the 
  evidence 
  afforded 
  by 
  a 
  nearly 
  entire 
  

   shell 
  of 
  the 
  last-named 
  species 
  from 
  the 
  Wealden, 
  it 
  appears 
  that 
  

   the 
  plastra 
  described 
  by 
  Sir 
  E. 
  Owen 
  as 
  Platemys 
  Mantelli 
  and 
  

   P. 
  Dixoni 
  are 
  really 
  referable 
  to 
  the 
  so-called 
  Chelone 
  Belli, 
  which 
  

   is 
  thus 
  shown 
  to 
  be 
  an 
  allied 
  form. 
  

  

  It 
  may 
  also 
  be 
  observed 
  in 
  this 
  connexion 
  that 
  the 
  so-called 
  

   Pleurosternum 
  latiscutatum 
  was 
  provisionally 
  referred 
  by 
  my 
  friend 
  

   and 
  myself 
  in 
  the 
  paper 
  above 
  cited 
  to 
  the 
  genus 
  Plesioclielys. 
  Subse- 
  

   quent 
  observations 
  have, 
  however, 
  shown 
  that 
  the 
  vertebral 
  shields 
  

   are 
  much 
  wider 
  than 
  in 
  that 
  genus 
  ; 
  and 
  since 
  there 
  are 
  other 
  

   distinctive 
  features 
  which 
  I 
  shall 
  indicate 
  elsewhere, 
  I 
  feel 
  jus- 
  

   tified 
  in 
  proposing 
  the 
  new 
  generic 
  name 
  Hylaioclielys 
  for 
  this 
  

   Chelonian. 
  I 
  may 
  state, 
  however, 
  that 
  I 
  have 
  found 
  it 
  impossible 
  

  

  * 
  G-eol. 
  Mag. 
  decade 
  3, 
  vol. 
  iv. 
  p. 
  272 
  (1887). 
  In 
  this 
  communication 
  

   Pleurosternum 
  was 
  referred 
  to 
  the 
  Pelomedusidae. 
  

  

  Q. 
  J. 
  G. 
  S. 
  No. 
  179. 
  2o 
  

  

  