﻿438 
  H. 
  G. 
  SEEIET 
  Otf 
  SOME 
  GENEEIC 
  MODIFICATIONS 
  

  

  On 
  the 
  ehelonian 
  ventral 
  surface 
  (fig. 
  3) 
  potential 
  growth 
  has 
  in 
  the 
  

   same 
  way 
  repeated, 
  in 
  the 
  connective 
  tissue, 
  the 
  girdle-ossifications, 
  

   which 
  were 
  subjected 
  most 
  energetically 
  to 
  the 
  intermittent 
  pressure 
  

   of 
  movement, 
  by 
  which 
  all 
  ossifications 
  are 
  originated, 
  extended, 
  and 
  

  

  Fig. 
  3. 
  — 
  Plastron 
  of 
  Embryonic 
  Testudo. 
  (After 
  Owen.) 
  

  

  CI. 
  Clavicle 
  of 
  authors 
  (potential 
  precoracoid). 
  /. 
  Interclavicle. 
  

   Hy. 
  Hyosternal. 
  Hp. 
  Hyposternal. 
  Ay. 
  Xyphisternal. 
  

  

  moulded. 
  Thus 
  the 
  hyposternal 
  and 
  xyphisternal 
  bones 
  (Bjp, 
  Xy) 
  

   reproduce 
  the 
  pubes 
  and 
  ischia 
  in 
  the 
  region 
  of 
  the 
  posterior 
  abdo- 
  

   minal 
  ribs 
  ; 
  and 
  in 
  certain 
  Chelonians 
  those 
  pelvic 
  bones 
  become 
  as 
  

   firmly 
  united 
  to 
  the 
  elements 
  of 
  the 
  plastron 
  beneath 
  them 
  as 
  the 
  

   epiphyses 
  of 
  a 
  mammalian 
  limb-bone 
  to 
  the 
  shaft 
  in 
  old 
  age. 
  In 
  the 
  

   same 
  way 
  the 
  coracoids 
  are 
  potentially 
  represented 
  by 
  the 
  hyosternal 
  

   bones 
  on 
  which 
  they 
  rest 
  (Hij.). 
  The 
  precoracoids 
  are 
  potentially 
  

   represented 
  by 
  the 
  pair 
  of 
  bones 
  which 
  have 
  been 
  variously 
  named 
  

   the 
  clavicles 
  or 
  episternal 
  bones 
  (CI). 
  The 
  ehelonian 
  interclavicle 
  

   or 
  entosternal 
  bone 
  (/) 
  has 
  no 
  prototype 
  in 
  the 
  ehelonian 
  pectoral 
  

   girdle, 
  and 
  is 
  probably 
  the 
  interclavicular 
  element 
  pressed 
  into 
  the 
  

   external 
  skeleton 
  by 
  the 
  precoracoids, 
  though 
  it 
  might 
  even 
  have 
  

   been 
  generated 
  kinetically 
  as 
  a 
  consequence 
  of 
  the 
  alternate 
  pressure 
  

   and 
  tension 
  of 
  those 
  ossifications 
  against 
  each 
  other 
  consequent 
  on 
  

   locomotion, 
  if 
  there 
  were 
  any 
  reason 
  for 
  supposing 
  the 
  interclavicle 
  

   absent. 
  

  

  These 
  remarks 
  upon 
  the 
  ehelonian 
  plastron 
  and 
  appendicular 
  girdles 
  

   seem 
  the 
  more 
  necessary, 
  because 
  the 
  Plesiosauria 
  show 
  no 
  sign 
  what- 
  

   ever 
  of 
  an 
  accumulation 
  of 
  organic 
  energy 
  in 
  their 
  organization 
  which 
  

   could 
  not 
  be 
  manifested 
  in 
  extension 
  of 
  the 
  vertebral 
  column 
  ; 
  and 
  

   we 
  look 
  in 
  those 
  animals 
  for 
  no 
  such 
  elements 
  as 
  characterize 
  the 
  ehe- 
  

   lonian 
  plastron, 
  precisely 
  because 
  we 
  find 
  empirically 
  that 
  the 
  energy 
  

   of 
  organization, 
  due 
  to 
  movement 
  of 
  the 
  vertebral 
  column, 
  expended 
  

   itself 
  in 
  adding 
  new 
  segments 
  to 
  the 
  body 
  ; 
  while 
  the 
  development 
  

   of 
  the 
  ehelonian 
  plastron 
  is 
  equally 
  in 
  harmony 
  with 
  the 
  specializa- 
  

   tion 
  of 
  locomotive 
  energy 
  in 
  the 
  limbs 
  : 
  but 
  since 
  the 
  girdle-bones 
  of 
  

   Plesiosaurs 
  occupy 
  similar 
  positions 
  to 
  the 
  pieces 
  of 
  the 
  ehelonian 
  plas- 
  

   tron, 
  and 
  the 
  similarity 
  of 
  the 
  limbs 
  implies 
  not 
  dissimilar 
  functions 
  

   for 
  the 
  abdominal 
  region 
  in 
  the 
  marine 
  groups 
  of 
  both 
  types, 
  I 
  attri- 
  

  

  