﻿514 
  ME. 
  R. 
  LYDEKKER 
  ON 
  CERTAIN 
  CHELONIAN 
  REMAINS 
  

  

  to 
  satisfy 
  myself 
  absolutely 
  that 
  this 
  form 
  is 
  generically 
  distinct 
  

   from 
  either 
  Hydropelta 
  of 
  the 
  Lithographic 
  Limestone 
  or 
  Ohitra- 
  

   cephalus 
  of 
  the 
  Wealden, 
  the 
  type 
  specimens 
  of 
  those 
  two 
  genera 
  

   being 
  of 
  a 
  character 
  which 
  does 
  not 
  admit 
  of 
  exact 
  comparison. 
  I 
  

   shall, 
  however, 
  elsewhere 
  adduce 
  certain 
  evidence 
  tending 
  to 
  show 
  

   that 
  Hylceochelys 
  is 
  not 
  identical 
  with 
  Ghitracephalus. 
  A 
  second 
  

   species 
  of 
  Hylceochelys 
  will 
  be 
  represented 
  by 
  Ohelone 
  Belli, 
  which 
  

   appears 
  to 
  be 
  specifically 
  distinct 
  from 
  the 
  type 
  species 
  : 
  and 
  in 
  

   any 
  case 
  the 
  specific 
  name 
  Belli, 
  as 
  the 
  earlier, 
  has 
  a 
  right 
  to 
  stand. 
  

   The 
  Chelonian 
  from 
  the 
  Kimeridgian 
  of 
  Hanover 
  described 
  by 
  

   Maack 
  * 
  as 
  Chelonides 
  Wittei 
  would 
  appear 
  to 
  indicate 
  a 
  form 
  more 
  

   or 
  less 
  closely 
  allied 
  to 
  Hylceochelys 
  ; 
  but 
  the 
  generic 
  name 
  is 
  pre- 
  

   occupied 
  f. 
  The 
  insufficiently 
  described 
  Plastremys, 
  Owen, 
  is 
  pro- 
  

   bably 
  also 
  identical 
  with 
  this 
  genus. 
  

  

  Reverting 
  to 
  the 
  forms 
  described 
  as 
  Pleurosternum, 
  it 
  is 
  evident 
  

   that 
  the 
  type 
  of 
  P. 
  emaryinatum, 
  Owen, 
  also 
  belongs 
  to 
  Hylceochelys, 
  

   although 
  the 
  other 
  examples 
  referred 
  to 
  that 
  species 
  by 
  its 
  founder 
  

   are 
  veritable 
  Pleurosternidae. 
  Again 
  the 
  specimen 
  from 
  the 
  Wealden 
  

   of 
  Germany 
  described 
  as 
  P. 
  Koeneni 
  % 
  is 
  likewise 
  referable 
  to 
  the 
  

   new 
  genus, 
  and 
  probably 
  belongs 
  to 
  the 
  type 
  species. 
  

  

  The 
  removal 
  of 
  these 
  three 
  species 
  from 
  Pleurosternum 
  will 
  reduce 
  

   the 
  four 
  species 
  assigned 
  to 
  that 
  genus 
  by 
  its 
  founder, 
  Sir 
  R. 
  Owen§, 
  

   to 
  two, 
  viz. 
  P. 
  continuum 
  and 
  P. 
  ovatum. 
  The 
  former 
  species 
  is 
  the 
  

   first 
  of 
  the 
  four 
  which 
  are 
  referred 
  to 
  the 
  genus 
  in 
  the 
  original 
  

   memoir, 
  and, 
  as 
  pointed 
  out 
  by 
  Prof. 
  Cope 
  ||, 
  must 
  undoubtedly 
  be 
  

   regarded 
  as 
  the 
  type 
  of 
  the 
  genus. 
  In 
  the 
  joint 
  communication 
  by 
  

   Mr. 
  Boulenger 
  and 
  myself 
  published 
  in 
  the 
  ' 
  Geological 
  Magazine/ 
  

   to 
  which 
  reference 
  has 
  been 
  already 
  made, 
  it 
  was 
  shown 
  that 
  the 
  

   plastron 
  described 
  by 
  Sir 
  11. 
  Owen 
  at 
  an 
  earlier 
  date 
  under 
  the 
  

   name 
  of 
  Platemys 
  Bullocki, 
  and 
  erroneously 
  supposed 
  to 
  have 
  been 
  

   obtained 
  from 
  the 
  London 
  Clay, 
  was 
  in 
  reality 
  from 
  the 
  Purbeck, 
  

   and 
  appeared 
  to 
  be 
  specifically 
  identical 
  with 
  the 
  type 
  of 
  Pleuro- 
  

   sternum 
  ovatum 
  and 
  with 
  some 
  of 
  the 
  specimens 
  described 
  as 
  

   P. 
  emaryinatum. 
  We 
  accordingly 
  proposed 
  to 
  supersede 
  the 
  name 
  

   Pleurosternum 
  ovatum 
  by 
  Pleurosternum 
  Bullocki, 
  making 
  no 
  mention 
  

   of 
  P. 
  concinnum. 
  It 
  had, 
  however, 
  escaped 
  our 
  notice 
  that 
  Prof. 
  

   Cope 
  ^[ 
  had 
  seen 
  occasion 
  to 
  regard 
  the 
  so-called 
  Platemys 
  Bul- 
  

   locki 
  as 
  generically 
  distinct 
  from 
  Pleurosternum 
  (tj^pified 
  by 
  P. 
  con- 
  

   cinnum), 
  and 
  had 
  proposed 
  for 
  it 
  the 
  name 
  Digerrhum. 
  

  

  It 
  appears, 
  indeed, 
  so 
  far 
  as 
  I 
  can 
  gather, 
  that 
  Prof. 
  Cope 
  was 
  

   induced 
  to 
  separate 
  Platemys 
  Bullocki 
  from 
  Pleurosternum 
  on 
  the 
  

   ground 
  that 
  the 
  latter 
  had 
  no 
  intergular 
  shield. 
  A 
  portion 
  of 
  such 
  

   shield 
  is, 
  however, 
  clearly 
  seen 
  in 
  P. 
  concinnum 
  — 
  the 
  type 
  of 
  the 
  

   latter 
  genus 
  — 
  and 
  the 
  distinction 
  consequently 
  falls 
  to 
  the 
  ground. 
  

  

  * 
  ' 
  Palaeontographica,' 
  vol. 
  xviii. 
  part 
  2, 
  p. 
  133 
  (1869). 
  

  

  t 
  In 
  1834 
  for 
  a 
  genus 
  of 
  Lepidoptera. 
  

  

  \ 
  Grabbe, 
  ' 
  Zeitschr. 
  deutsch. 
  geol. 
  Gres.' 
  vol. 
  xxxvi. 
  p. 
  19 
  (1881). 
  

  

  § 
  « 
  Wealden 
  and 
  Purbeck 
  Eeptilia 
  ' 
  (Mon. 
  Pal. 
  Soc), 
  pt. 
  i. 
  p. 
  2 
  (1853). 
  

  

  || 
  Geol. 
  Mag. 
  decade 
  3, 
  vol. 
  iv. 
  p. 
  573 
  (1887). 
  

  

  % 
  Trans. 
  Amer. 
  Phil. 
  Soc. 
  vol. 
  xiv. 
  pt. 
  i. 
  p. 
  156 
  (1870). 
  

  

  