﻿182 
  

  

  PKOF. 
  OWEN 
  ON 
  AN 
  EXTINCT 
  CHELONIAN 
  

  

  Pig, 
  2. 
  Anterior 
  Portion 
  of 
  the 
  Plastron 
  of 
  Notochelys 
  costata 
  

  

  (J 
  nat. 
  size). 
  

  

  hpt.r 
  

  

  In 
  both 
  elements 
  of 
  the 
  plastron 
  the 
  exposed 
  surface 
  has 
  lost 
  some 
  

   substance 
  near 
  the 
  mesial 
  margin. 
  Sufficient, 
  however, 
  remains 
  to 
  

   throw 
  welcome 
  light 
  on 
  the 
  affinities 
  of 
  this 
  Chelonite. 
  

  

  The 
  characters 
  of 
  the 
  carapace 
  might 
  be 
  interpreted 
  as 
  those 
  of 
  a 
  

   true 
  Turtle 
  (Chelone) 
  not 
  modified 
  sufficiently 
  to 
  bear 
  a 
  subgeneric 
  

   distinction. 
  But 
  those 
  of 
  the 
  plastron 
  show 
  the 
  well-marked 
  cha- 
  

   racters 
  of 
  the 
  part 
  in 
  Trionyx 
  and 
  Chelys 
  ; 
  the 
  hyosternal 
  and 
  hy- 
  

   posternal, 
  which 
  are 
  separated 
  by 
  a 
  persistent 
  transverse 
  suture 
  in 
  

   Chelone 
  as 
  in 
  Emys 
  and 
  Testudo, 
  have 
  coalesced, 
  and 
  so 
  completely 
  in 
  

   Notochelys 
  as 
  to 
  leave 
  no 
  trace 
  of 
  the 
  original 
  presence 
  of 
  the 
  suture 
  in 
  

   the 
  immature 
  reptile. 
  The 
  hyosternal 
  element 
  in 
  Chelonians 
  where 
  

   the 
  plastron 
  is 
  best 
  ossified, 
  as 
  in 
  the 
  extinct 
  Eocene 
  kinds, 
  does 
  not 
  

   extend 
  backward 
  beyond 
  the 
  second 
  vertebral 
  scute 
  on 
  the 
  fourth 
  

   neural 
  plate 
  * 
  ; 
  whereas 
  the 
  single 
  plastral 
  bones 
  (fig. 
  2, 
  hjos), 
  

   extend 
  backward 
  beyond 
  the 
  third 
  vertebral 
  scute 
  (fig. 
  1, 
  v 
  3), 
  and 
  

   probably 
  beyond 
  the 
  fifth 
  neural 
  plate 
  ; 
  for 
  the 
  hindmost 
  angles 
  of 
  

   both 
  plastral 
  bones 
  have 
  suffered 
  fracture. 
  Sufficient 
  is 
  preserved, 
  

   laterally, 
  to 
  show 
  that 
  the 
  carapace 
  and 
  plastron 
  were 
  not 
  united 
  

  

  * 
  Compare 
  Hist, 
  of 
  Brit. 
  Foss. 
  Kept. 
  pi. 
  I 
  (Chelone 
  breviceps), 
  pi. 
  13 
  (Chelone 
  

   longiceps), 
  pi. 
  14 
  (Chelone 
  convexa). 
  See 
  also 
  Cuvier, 
  Ossemens 
  Fossiles, 
  torn. 
  v. 
  

   pt. 
  ii. 
  (4to, 
  1824) 
  pi. 
  xiii. 
  fig. 
  6 
  (Chelone 
  mydas), 
  fig. 
  7 
  (Chelone 
  caretta). 
  

  

  