﻿58 
  Dr. 
  G. 
  Baur 
  on 
  the 
  

  

  not 
  exist 
  in 
  Meiolania, 
  and 
  there 
  can 
  be 
  no 
  doubt 
  whatever 
  

   that 
  Meiolania 
  is 
  a 
  true 
  Gryptodiran 
  *. 
  

  

  The 
  question 
  now 
  arises, 
  to 
  which 
  group 
  of 
  the 
  Ciyptodira 
  

   does 
  Meiolania 
  belong 
  ? 
  Prof. 
  Huxley 
  says 
  that 
  it 
  is 
  " 
  closely 
  

   allied 
  to 
  the 
  living 
  Chelydra, 
  Gypochelys, 
  and 
  Platy 
  sternum." 
  

  

  But 
  the 
  Platysrernidae 
  have 
  nothing 
  in 
  common 
  with 
  the 
  

   Chelydridae 
  ; 
  they 
  belong 
  to 
  quite 
  a 
  different 
  group 
  together 
  

   with 
  the 
  Testudinidae 
  and 
  Emydidae. 
  Therefore 
  Meiolania 
  

   cannot 
  be 
  genetically 
  allied 
  to 
  both. 
  

  

  The 
  only 
  similarity 
  between 
  the 
  Platysternidae 
  and 
  Chely- 
  

   dridae 
  consists 
  in 
  the 
  appearance 
  of 
  the 
  skull 
  and 
  the 
  opistho- 
  

   coelian 
  nature 
  of 
  a 
  part 
  of 
  the 
  caudal 
  vertebra. 
  Both 
  these 
  

   similarities, 
  however, 
  are 
  the 
  result 
  of 
  parallelism 
  only. 
  

  

  The 
  opisthoccelian 
  caudal 
  vertebra? 
  are 
  of 
  secondary 
  nature. 
  

  

  All 
  Testudinata 
  have 
  the 
  second 
  sacral 
  vertebra 
  convex 
  

   behind. 
  There 
  is 
  no 
  exception 
  whatever. 
  Prof. 
  Leidy 
  f, 
  it 
  

   is 
  true, 
  states 
  that 
  " 
  the 
  posterior 
  articular 
  surface 
  of 
  the 
  

   second 
  [sacral] 
  centrum 
  is 
  concave 
  " 
  in 
  Bcena. 
  But 
  this 
  is 
  

   not 
  correct 
  : 
  what 
  Prof. 
  Leidy 
  describes 
  as 
  the 
  posterior 
  arti- 
  

   cular 
  surface 
  of 
  the 
  second 
  is 
  the 
  anterior 
  of 
  the 
  first 
  sacral 
  

   vertebra. 
  The 
  second 
  sacral 
  vertebra 
  of 
  Bcena 
  is 
  convex 
  

   behind, 
  as 
  in 
  all 
  other 
  Testudinata. 
  Therefore 
  the 
  first 
  

   caudal 
  vertebra 
  must 
  be 
  either 
  concave-convex 
  (procoelous) 
  or 
  

   biconcave, 
  never 
  convex-concave 
  (opisthoccelous) 
  or 
  biconvex. 
  

   In 
  fact 
  it 
  is 
  always 
  jjrocoelous, 
  and 
  so 
  are 
  the 
  next 
  following 
  

   vertebra?. 
  Now 
  if 
  opisthoccelian 
  vertebras 
  are 
  found 
  in 
  the 
  

   caudals 
  of 
  some 
  Chelonians, 
  it 
  can 
  only 
  be 
  produced 
  by 
  the 
  

   interference 
  of 
  an 
  amphiccelian 
  vertebra 
  ; 
  and 
  this 
  is 
  the 
  case 
  

   in 
  Chelydra, 
  Macrochelys, 
  Platy 
  sternum, 
  and 
  must 
  also 
  be 
  in 
  

   Bcena 
  and 
  Meiolania. 
  

  

  In 
  Platysternum 
  (one 
  specimen) 
  the 
  fourth 
  caudal 
  vertebra 
  

   is 
  biconcave, 
  in 
  Chelydra 
  and 
  Macrochelys 
  the 
  third 
  or 
  fourth. 
  

   Prof. 
  Huxley 
  thinks 
  that 
  perhaps 
  Staurotypus 
  may 
  also 
  have 
  

   opisthoccelian 
  vertebrae 
  ; 
  but 
  this 
  is 
  not 
  the 
  case. 
  

  

  That 
  the 
  opisthoccelian 
  caudal 
  vertebrae 
  of 
  Chelonians 
  are 
  

   of 
  secondary 
  nature 
  is 
  also 
  proved 
  by 
  palaeontology. 
  In 
  

   Toxochelys, 
  Cope, 
  an 
  indubitable 
  Chelydroid 
  J 
  from 
  the 
  

   Cretaceous, 
  the 
  caudal 
  vertebrae 
  are 
  procoeluus. 
  

  

  * 
  Another 
  support 
  of 
  this 
  view 
  is 
  given 
  by 
  the 
  caudal 
  vertebra? 
  ; 
  there 
  

   are 
  well-developed 
  paradiapophyses 
  in 
  the 
  caudals 
  of 
  the 
  Pleurodira 
  ; 
  in 
  

   Meiolania 
  we 
  have 
  well-developed 
  caudal 
  ribs 
  (as 
  in 
  Cryptudira) 
  coossiiied 
  

   with 
  the 
  centrum. 
  

  

  t 
  Leidy, 
  Jos., 
  ' 
  Contributions 
  to 
  the 
  Extinct 
  Vertebrate 
  Fauna 
  of 
  the 
  

   Western 
  Territories,' 
  Washington, 
  V., 
  1873, 
  p. 
  108. 
  

  

  \ 
  I 
  may 
  notice 
  here 
  that 
  Anosteira, 
  Leidy, 
  placed 
  by 
  Cope 
  among 
  the 
  

   Chelydridae 
  and 
  by 
  Boulenger 
  among 
  the 
  " 
  Pseudotrionychidae," 
  belongs 
  

  

  