﻿FKOM 
  THE 
  WEALDEN 
  AND 
  PUBJBECK. 
  515 
  

  

  At 
  one 
  time 
  I 
  thought 
  that 
  the 
  undermentioned 
  specimens 
  might 
  on 
  

   other 
  grounds 
  justify 
  the 
  retention 
  of 
  Digerrhum 
  as 
  a 
  form 
  closely 
  

   allied 
  to 
  Pleurostemum, 
  but 
  further 
  consideration 
  induced 
  me 
  to 
  

   regard 
  all 
  the 
  above-mentioned 
  forms 
  as 
  referable 
  to 
  one 
  genus, 
  and 
  

   probably 
  to 
  a 
  single 
  species, 
  for 
  which 
  the 
  name 
  Pleurostemum 
  Bul- 
  

   locli 
  should 
  be 
  adopted. 
  

  

  It 
  may 
  be 
  added 
  that 
  while 
  Pleurostemum 
  was, 
  from 
  the 
  assumed 
  

   absence 
  of 
  the 
  intergular, 
  referred 
  by 
  Prof. 
  Cope 
  to 
  the 
  Cryptodira 
  

   as 
  the 
  type 
  of 
  a 
  family, 
  Digerrhum 
  was 
  classed 
  among 
  the 
  Pleurodira 
  

   in 
  the 
  existing 
  family 
  Stern 
  othseridge, 
  which 
  is 
  included 
  by 
  Mr. 
  

   Boulenger 
  among 
  the 
  Pelomedusidae. 
  

  

  Having 
  now 
  cleared 
  the 
  ground, 
  we 
  may 
  proceed 
  to 
  consider 
  the 
  

   specimens 
  which 
  I 
  have 
  to 
  bring 
  to 
  notice 
  as 
  affording 
  evidence 
  of 
  

   the 
  affinities 
  of 
  Pleurostemum. 
  It 
  may 
  be 
  observed 
  in 
  the 
  first 
  

   place 
  that 
  the 
  most 
  distinctive 
  feature 
  of 
  this 
  genus 
  is 
  the 
  presence 
  

   of 
  a 
  complete 
  mesoplastral 
  element 
  in 
  the 
  plastron 
  (fig. 
  3), 
  this 
  feature 
  

   occurring 
  elsewhere, 
  so 
  far 
  as 
  is 
  known, 
  only 
  in 
  the 
  allied 
  Helochelys 
  

   of 
  the 
  Neocomian, 
  in 
  the 
  existing 
  Pleurodiran 
  genus 
  Sternoihcerus, 
  

   and 
  possibly 
  in 
  the 
  Triassic 
  Proganochelys 
  ; 
  while 
  such 
  an 
  element, 
  

   although 
  of 
  a 
  different 
  type, 
  is 
  considered 
  to 
  have 
  been 
  probably 
  

   developed 
  in 
  the 
  Wealden 
  form 
  described 
  in 
  the 
  first 
  part 
  of 
  this 
  

   communication. 
  

  

  Further 
  the 
  shell 
  is 
  comparatively 
  smooth, 
  and 
  has 
  an 
  intergular 
  

   but 
  no 
  nuchal 
  epidermal 
  shield 
  ; 
  while 
  the 
  entoplastral 
  is 
  wide 
  and 
  

   of 
  relatively 
  large 
  size. 
  There 
  is, 
  moreover, 
  a 
  full 
  series 
  of 
  neural 
  

   bones, 
  of 
  which 
  the 
  8th 
  articulates 
  with 
  the 
  1st 
  suprapygal 
  ; 
  while 
  

   the 
  vertebral 
  shields 
  are 
  relatively 
  wide. 
  

  

  The 
  first 
  of 
  the 
  two 
  specimens 
  I 
  have 
  to 
  bring 
  under 
  the 
  notice 
  of 
  

   the 
  Society 
  is 
  a 
  small 
  slab 
  of 
  rock 
  (B.M. 
  No. 
  48262), 
  showing 
  the 
  

   greater 
  part 
  of 
  the 
  flattened 
  shell 
  of 
  an 
  immature 
  Chelonian. 
  This 
  

   specimen, 
  which 
  is 
  represented 
  of 
  two 
  thirds 
  the 
  natural 
  size 
  in 
  fig. 
  3, 
  

   shows 
  the 
  greater 
  part 
  of 
  the 
  median 
  line 
  and 
  of 
  the 
  left 
  half 
  of 
  the 
  

   carapace, 
  the 
  right 
  half 
  of 
  the 
  latter 
  having 
  been 
  chiselled 
  away 
  in 
  

   order 
  to 
  exhibit 
  the 
  dorsal 
  surface 
  of 
  the 
  plastron. 
  The 
  general 
  

   contour 
  of 
  the 
  specimen, 
  the 
  absence 
  of 
  the 
  nuchal 
  shield, 
  and 
  more 
  

   especially 
  the 
  complete 
  mesoplastral 
  bones, 
  at 
  once 
  indicate 
  that 
  

   it 
  belongs 
  to 
  the 
  Pleurosternidae. 
  It 
  will 
  be 
  seen 
  from 
  the 
  figure 
  

   that 
  the 
  first 
  marginal 
  bone 
  of 
  either 
  side 
  encroaches 
  so 
  largely 
  on 
  

   the 
  anterior 
  border 
  of 
  the 
  nuchal, 
  as 
  to 
  leave 
  scarcely 
  any 
  free 
  

   border 
  to 
  that 
  bone 
  : 
  and 
  I 
  was 
  at 
  first 
  inclined 
  to 
  consider 
  this 
  a 
  

   specific 
  distinction 
  from 
  Pleurostemum 
  Bullocki. 
  Finding, 
  however, 
  

   that 
  the 
  same 
  feature 
  occurs 
  in 
  another 
  young 
  carapace 
  (and, 
  indeed, 
  

   in 
  all 
  the 
  young 
  specimens 
  in 
  the 
  Museum), 
  while 
  all 
  the 
  adult 
  speci- 
  

   mens 
  show 
  a 
  more 
  normal 
  type 
  of 
  nuchal, 
  I 
  have 
  finally 
  come 
  to 
  the 
  

   conclusion 
  that 
  the 
  feature 
  obtaining 
  in 
  the 
  nuchal 
  of 
  the 
  specimen 
  

   under 
  consideration 
  should 
  probably 
  be 
  regarded 
  as 
  characteristic 
  

   of 
  immaturity. 
  It 
  should, 
  however, 
  be 
  observed 
  that 
  in 
  Sir 
  li. 
  

   Owen's 
  figure 
  of 
  the 
  type 
  specimen 
  of 
  P. 
  concinnum, 
  the 
  first 
  mar- 
  

   ginals 
  appear 
  to 
  have 
  a 
  somewhat 
  similar 
  relation 
  to 
  the 
  nuchal 
  as 
  

   obtains 
  in 
  the 
  present 
  young 
  specimen 
  ; 
  but 
  an 
  inspection 
  of 
  the 
  

  

  2o2 
  

  

  