﻿228 
  ME. 
  E. 
  LYDEKKEE 
  OX 
  BEIOIXS 
  OF 
  EOCENE 
  AXD 
  HESOZOIC 
  

  

  frontal 
  distinct 
  ; 
  by 
  the 
  posterior 
  nares 
  being 
  formed 
  by 
  the 
  maxillary 
  

   and 
  palatine 
  bones, 
  and 
  divided 
  by 
  the 
  whole 
  length 
  of 
  the 
  vomer, 
  

   which 
  extends 
  on 
  the 
  palate 
  between 
  the 
  palatine 
  and 
  prein 
  axillary 
  

   bones 
  ; 
  and 
  by 
  the 
  temporal 
  region 
  being 
  covered 
  by 
  an 
  arrangement 
  

   of 
  bones 
  like 
  that 
  in 
  Clielone." 
  This 
  description 
  differs 
  from 
  Owen's 
  

   figure 
  by 
  the 
  introduction 
  of 
  the 
  vomer, 
  which 
  is 
  not 
  shown 
  in 
  

   the 
  latter. 
  Although 
  I 
  have 
  not 
  seen 
  specimens 
  showing 
  the 
  presence 
  

   of 
  the 
  vomer, 
  I 
  take 
  it, 
  on 
  theoretical 
  grounds, 
  that 
  this 
  element 
  was 
  

   probably 
  present. 
  There 
  is 
  unfortunately 
  a 
  little 
  ambiguity 
  in 
  the 
  

   expression 
  employed 
  by 
  Prof. 
  Seeley 
  as 
  to 
  the 
  vomer 
  extending 
  

   between 
  the 
  palatines 
  and 
  the 
  premaxillge, 
  as 
  this 
  might 
  mean 
  

   separating 
  the 
  two 
  palatines 
  from 
  one 
  another 
  ; 
  I 
  take 
  it, 
  however, 
  

   from 
  the 
  evidence 
  of 
  Owen's 
  figure, 
  that 
  it 
  is 
  intended 
  to 
  mean 
  that 
  

   the 
  vomer 
  connects 
  the 
  palatines 
  with 
  the 
  premaxilla?. 
  

  

  In 
  1873 
  Prof. 
  S. 
  Eiitimeyer,who 
  appears 
  to 
  have 
  been 
  unacquainted 
  

   with 
  the 
  name 
  Bhinochelys, 
  on 
  pp. 
  148-150 
  of 
  his 
  ' 
  Fossile 
  Schild- 
  

   kroten 
  von 
  Solothurn,' 
  came 
  to 
  the 
  conclusion 
  that 
  the 
  skull 
  of 
  

   Chelone 
  jmlcliriceps 
  showed, 
  on 
  the 
  whole, 
  more 
  affinities 
  with 
  the 
  

   Chelydidas 
  than 
  with 
  any 
  other 
  family, 
  and 
  concluded 
  that 
  this 
  form 
  

   probably 
  indicated 
  a 
  Pleurodiran. 
  

  

  With 
  this 
  conclusion 
  I 
  am 
  disposed 
  to 
  agree, 
  since, 
  if 
  we 
  put 
  aside 
  

   the 
  alleged 
  occurrence 
  of 
  separate 
  nasals 
  in 
  the 
  so-called 
  Euclastes, 
  

   the 
  only 
  existing 
  Chelonians 
  having 
  distinct 
  nasals 
  are 
  the 
  Chelydida? 
  ; 
  

   while, 
  with 
  the 
  exception 
  of 
  the 
  Trionychoidea, 
  it 
  is 
  only 
  among 
  the 
  

   Pelomedusidse 
  that 
  we 
  find 
  the 
  palatines 
  uniting 
  in 
  the 
  middle 
  line. 
  

   The 
  anterior 
  portion 
  of 
  the 
  palate 
  of 
  Bliinochelys 
  has, 
  indeed, 
  the 
  same 
  

   relations 
  of 
  the 
  bones 
  surrounding 
  the 
  posterior 
  nares 
  as 
  in 
  the 
  existing 
  

   Pelomedusa*, 
  with 
  the 
  exception 
  that 
  in 
  the 
  latter 
  the 
  vomer 
  is 
  

   absent. 
  It 
  is 
  true 
  that 
  the 
  palatines 
  of 
  Rliinoclielys 
  are 
  much 
  nar- 
  

   rower 
  than 
  in 
  any 
  recent 
  Pleurodiran, 
  and 
  are 
  thereby 
  of 
  a 
  Crypto- 
  

   diran 
  type 
  ; 
  while 
  the 
  complete 
  roofing 
  over 
  of 
  the 
  temporal 
  fossae 
  

   is 
  not 
  found 
  in 
  any 
  recent 
  Pleurodira, 
  the 
  roof 
  found 
  in 
  Podocnemis 
  

   being 
  of 
  a 
  totally 
  different 
  structure. 
  I 
  am, 
  however, 
  disposed 
  to 
  

   consider 
  that 
  the 
  extremely 
  wide 
  pterygoids 
  of 
  existing 
  Pleurodirans, 
  

   together 
  with 
  the 
  remarkable 
  structure 
  of 
  the 
  tympanic 
  ring 
  and 
  the 
  

   mandibular 
  articulation, 
  are 
  characters 
  more 
  or 
  less 
  recently 
  acquired 
  ; 
  

   while 
  the 
  complete 
  bony 
  roof 
  of 
  Rliinoclielys 
  I 
  look 
  upon 
  as 
  an 
  archaic 
  

   character, 
  since 
  it 
  occurs 
  in 
  many 
  of 
  the 
  Jurassic 
  Chelonians, 
  some 
  

   of 
  which, 
  like 
  Eurysternum, 
  are 
  Cryptodiran, 
  while 
  others, 
  like 
  Idio- 
  

   clielys, 
  are 
  probably 
  Pleurodiran. 
  Remnants 
  of 
  this 
  roof 
  are, 
  I 
  think, 
  

   to 
  be 
  found 
  in 
  the 
  parieto-squamosal 
  bar 
  of 
  many 
  Chelydidae. 
  My 
  

   reasons 
  for 
  regarding 
  the 
  broad 
  pterygoids 
  of 
  recent 
  Pleurodirans 
  as 
  

   an 
  acquired 
  character 
  is 
  that 
  all 
  Jurassic 
  Chelonian 
  skulls 
  with 
  which 
  

   we 
  are 
  acquainted 
  have 
  more 
  or 
  less 
  narrow 
  pterygoids, 
  and 
  it 
  is 
  

   practically 
  certain 
  that 
  some 
  of 
  these 
  are 
  Pleurodiran. 
  This 
  is 
  well 
  

   shown 
  in 
  the 
  skull 
  figured 
  in 
  pi. 
  xiv. 
  figs. 
  1, 
  2, 
  of 
  the 
  above- 
  cited 
  

   memoir 
  of 
  Prof. 
  Paitimeyer, 
  which 
  is 
  referred 
  with 
  considerable 
  pro- 
  

   bability 
  to 
  PlesiocJielys. 
  In 
  that 
  skull 
  the 
  vomer 
  divides 
  the 
  palatines, 
  

  

  * 
  See 
  Boulenger, 
  ' 
  Catalogue 
  of 
  Chelonians 
  &e. 
  in 
  the 
  British 
  Museum,' 
  p. 
  198, 
  

   fig. 
  48 
  (1889). 
  

  

  