﻿242 
  M&. 
  R. 
  LTDEKKEE 
  ON 
  REMAINS 
  OE 
  EOCENE 
  AND 
  MESOZOIC 
  

  

  level 
  of 
  either 
  of 
  the 
  two 
  ridges, 
  which 
  is 
  evidently 
  the 
  representa- 
  

   tive 
  of 
  the 
  much 
  smaller 
  process 
  found 
  at 
  the 
  hinder 
  border 
  of 
  the 
  

   mandibular 
  symphysis 
  of 
  the 
  Hawksbill 
  Turtle. 
  

  

  No 
  existing 
  Chelonian 
  has 
  anything 
  like 
  this 
  extraordinary 
  deve- 
  

   lopment 
  of 
  this 
  process 
  as 
  seen 
  in 
  the 
  present 
  specimen 
  ; 
  and 
  it 
  is 
  

   quite 
  evident 
  that 
  the 
  cranium 
  of 
  the 
  form 
  to 
  which 
  this 
  mandible 
  

   pertained 
  must 
  have 
  had 
  a 
  deep 
  pit 
  on 
  the 
  oral 
  surface 
  of 
  the 
  palate 
  

   for 
  the 
  reception 
  of 
  the 
  spine-like 
  process. 
  It 
  may 
  further 
  be 
  con- 
  

   cluded 
  that 
  our 
  specimen 
  indicates 
  a 
  Chelonian 
  of 
  herbivorous 
  habits. 
  

  

  The 
  Chelonians 
  having 
  serrated 
  alveolar 
  borders 
  to 
  the 
  mandible 
  

   having 
  been 
  mentioned, 
  it 
  will 
  be 
  well 
  to 
  show 
  how 
  they 
  differ 
  from 
  

   our 
  specimen. 
  Firstly, 
  with 
  regard 
  to 
  Testudo 
  elephantina, 
  the 
  sym- 
  

   physis 
  is 
  so 
  much 
  shorter, 
  and 
  the 
  whole 
  contour 
  is 
  so 
  different, 
  that 
  

   it 
  is 
  quite 
  clear 
  that 
  our 
  specimen 
  does 
  not 
  indicate 
  a 
  type 
  in 
  any 
  

   way 
  allied. 
  Compared 
  with 
  the 
  Batagurs 
  (Hardella, 
  Kachuga, 
  &c.) 
  

   there 
  is 
  somewhat 
  more 
  resemblance 
  ; 
  but 
  in 
  that 
  group 
  the 
  inner 
  

   ridge 
  of 
  the 
  symphysis 
  is 
  always 
  lower 
  than 
  the 
  outer, 
  and 
  in 
  neither 
  

   of 
  the 
  genera 
  is 
  the 
  contour 
  of 
  the 
  whole 
  symphysis 
  like 
  the 
  present 
  

   specimen. 
  In 
  the 
  Carnivorous 
  Testudinidse 
  the 
  mandibular 
  sym- 
  

   physis 
  differs 
  very 
  widely 
  from 
  the 
  latter. 
  

  

  Among 
  existing 
  forms 
  the 
  type 
  of 
  mandible 
  which 
  seems 
  to 
  come 
  

   nearest 
  to 
  the 
  fossil 
  is 
  that 
  of 
  the 
  genus 
  Podocnemis, 
  the 
  sole 
  existing 
  

   herbivorous 
  genus 
  among 
  the 
  Pleurodira. 
  This 
  resemblance 
  is 
  

   noticeable 
  in 
  the 
  whole 
  general 
  contour 
  of 
  the 
  symphysis, 
  and 
  espe- 
  

   cially 
  in 
  the 
  circumstance 
  that 
  the 
  inner 
  ridge 
  is 
  higher 
  than 
  the 
  

   outer. 
  It 
  is 
  true 
  that 
  Podocnemis 
  has 
  no 
  serrations 
  on 
  the 
  ridges, 
  

   nor 
  a 
  median 
  process 
  like 
  that 
  of 
  the 
  present 
  specimen, 
  but 
  these 
  

   might 
  be 
  merely 
  generic 
  differences. 
  

  

  Having, 
  then, 
  decided 
  that 
  the 
  specimen 
  under 
  consideration 
  indi- 
  

   cates 
  a 
  large 
  Chelonian 
  generically 
  distinct 
  from 
  any 
  existing 
  type, 
  

   but 
  apparently 
  showing 
  resemblances 
  to 
  Podocnemis, 
  we 
  have 
  to 
  

   face 
  the 
  problem 
  whether 
  it 
  can 
  be 
  identified 
  with 
  any 
  of 
  the 
  forms 
  

   hitherto 
  described 
  from 
  the 
  London 
  Clay. 
  Now 
  the 
  Chelonia 
  of 
  

   those 
  beds 
  comprise, 
  in 
  the 
  first 
  place, 
  certain 
  Dermatochelydida?, 
  

   Chelonidse, 
  and 
  Trionychidae 
  ; 
  and 
  it 
  is 
  quite 
  clear 
  that 
  our 
  speci- 
  

   men 
  has 
  no 
  affinity 
  with 
  either 
  of 
  these 
  three 
  families. 
  Next 
  we 
  

   have 
  the 
  genus 
  Pseudotrionyx, 
  of 
  which 
  the 
  full 
  affinities 
  are 
  un- 
  

   known, 
  but 
  which 
  appears 
  to 
  be 
  more 
  or 
  less 
  closely 
  allied 
  to 
  the 
  

   carnivorous 
  Chelydridae. 
  If 
  this 
  view 
  be 
  correct, 
  there 
  will 
  be 
  little 
  

   doubt 
  that 
  our 
  specimen 
  is 
  not 
  referable 
  to 
  Pseudotrionyce. 
  

  

  There 
  is, 
  however, 
  more 
  satisfactory 
  evidence 
  to 
  show 
  that 
  the 
  

   present 
  specimen 
  is 
  not 
  referable 
  to 
  Pseudotrionyce. 
  The 
  imperfect 
  

   skull 
  figured 
  in 
  plate 
  xxxix. 
  figs. 
  1, 
  2, 
  of 
  Owen's 
  ' 
  Eeptilia 
  of 
  the 
  

   London 
  Clay,' 
  and 
  provisionally 
  referred 
  to 
  the 
  Pleurodiran 
  genus 
  

   Platemys, 
  has 
  recently 
  been 
  cleaned 
  from 
  matrix, 
  and 
  found 
  not 
  only 
  

   to 
  be 
  Cryptodiran, 
  but 
  to 
  be 
  essentially 
  Chelydroid. 
  Now, 
  since 
  

   Pseudotrionyce 
  is 
  Chelydroid, 
  it 
  is 
  practically 
  certain 
  that 
  this 
  skull 
  

   belongs 
  to 
  that 
  genus. 
  The 
  alveolar 
  margins 
  of 
  that 
  specimen 
  are 
  

   entire, 
  and 
  the 
  whole 
  character 
  of 
  the 
  skull 
  agrees 
  very 
  closely 
  in 
  

   general 
  features 
  with 
  that 
  of 
  the 
  existing 
  Macroclemmys. 
  

  

  