﻿OF 
  THE 
  PLESIOSAtTRIAN 
  PECTOBAL 
  ARCH. 
  449 
  

  

  slightly 
  constricted. 
  No 
  mention 
  is 
  made 
  of 
  chevron 
  bones 
  in 
  the 
  

   caudal 
  vertebrae. 
  There 
  are 
  only 
  six 
  bones, 
  in 
  two 
  rows, 
  in 
  the 
  

   carpus 
  and 
  in 
  the 
  tarsus, 
  and 
  only 
  four 
  digits 
  in 
  each 
  limb. 
  The 
  

   premaxillary 
  bones 
  appear 
  to 
  extend 
  backward 
  so 
  as 
  to 
  divide 
  the 
  

   nasal 
  bones 
  ; 
  and 
  the 
  lower 
  jaw 
  is 
  unusually 
  deep 
  at 
  the 
  coronoid 
  

   bone. 
  The 
  double 
  articulation 
  for 
  the 
  cervical 
  rib 
  probably 
  indi- 
  

   cated 
  relationship 
  to 
  the 
  Pliosauridae 
  rather 
  than 
  to 
  either 
  of 
  the 
  

   families 
  that 
  I 
  have 
  discussed 
  ; 
  but 
  it 
  is 
  seen 
  in 
  isolated 
  vertebrae 
  

   from 
  the 
  Lias, 
  contained 
  in 
  the 
  British 
  Museum. 
  

  

  I 
  cannot 
  but 
  consider 
  it 
  a 
  matter 
  for 
  regret 
  that, 
  although 
  so 
  large 
  

   a 
  number 
  of 
  species 
  have 
  been 
  found 
  in 
  the 
  Cambridge 
  Upper 
  Green- 
  

   sand 
  and 
  other 
  Cretaceous 
  strata, 
  we 
  are 
  still 
  ignorant 
  of 
  all 
  those 
  

   parts 
  of 
  the 
  body 
  which 
  would 
  warrant 
  us 
  in 
  placing 
  them 
  in 
  genera. 
  

   One 
  is 
  thus 
  unable 
  to 
  pronounce 
  any 
  opinion 
  on 
  the 
  evolution 
  of 
  

   modifications 
  of 
  the 
  Plesiosaurian 
  girdle 
  in 
  relation 
  to 
  time. 
  And 
  

   the 
  only 
  general 
  conclusion 
  at 
  which 
  I 
  have 
  arrived 
  is 
  that 
  Plesio- 
  

   sauria, 
  in 
  common 
  with 
  all 
  similar 
  groups, 
  show, 
  in 
  the 
  newer 
  rocks 
  

   as 
  compared 
  with 
  the 
  older 
  ones, 
  a 
  greater 
  amount 
  of 
  ossific 
  energy, 
  

   probably 
  coincident 
  with 
  higher 
  organization, 
  which 
  manifests 
  itself 
  

   in 
  more 
  perfect 
  ossification 
  of 
  the 
  bones, 
  elongation 
  of 
  processes, 
  and 
  

   blending 
  of 
  subordinate 
  with 
  the 
  principal 
  ossific 
  centres. 
  

  

  