﻿OF 
  THE 
  PLESIOSAUEIAN 
  PECTOKAX 
  AECH. 
  437 
  

  

  the 
  precoracoids 
  and 
  coracoids, 
  and 
  a 
  thin 
  film 
  of 
  ossified 
  connective 
  

   tissue 
  extend 
  over 
  the 
  interspace 
  between 
  them 
  and 
  unite 
  these 
  

   bones 
  respectively 
  on 
  each 
  side 
  of 
  the 
  animal, 
  a 
  pair 
  of 
  coracoid 
  

   bones 
  will 
  be 
  constituted 
  similar 
  in 
  form, 
  position, 
  and 
  relation 
  to 
  

   those 
  which 
  characterize 
  the 
  Plesiosauria, 
  the 
  only 
  difference 
  being 
  

   that 
  in 
  Plesiosaurs 
  the 
  precoracoid 
  is 
  connate 
  with 
  the 
  coracoid, 
  as 
  

   in 
  the 
  Ostrich 
  and 
  many 
  Lizards 
  ; 
  while 
  in 
  Chelonians 
  it 
  is 
  connate 
  

   with 
  the 
  scapula 
  (s) 
  : 
  but 
  much 
  importance 
  cannot 
  be 
  attached 
  to 
  the 
  

   condition 
  of 
  the 
  precoracoid, 
  since 
  no 
  one 
  will 
  contend 
  that 
  Plesio- 
  

   saurs 
  are 
  either 
  Lizards 
  or 
  Tortoises 
  ; 
  while 
  among 
  Amphibians 
  the 
  

   precoracoid 
  is 
  a 
  distinct 
  bone, 
  so 
  placed 
  that 
  it 
  might 
  combine 
  indif- 
  

   ferently 
  with 
  coracoid 
  or 
  scapula. 
  

  

  It 
  is 
  a 
  noteworthy 
  fact, 
  familiar 
  to 
  all 
  who 
  collect 
  Plesiosaurian 
  

   coracoids 
  from 
  soft 
  strata, 
  that 
  the 
  whole 
  of 
  this 
  triangular 
  inter- 
  

   space 
  between 
  the 
  precoracoid 
  and 
  coracoid 
  parts, 
  which 
  I 
  covered 
  

   in 
  the 
  Turtle 
  with 
  an 
  imaginary 
  thin 
  ossification, 
  is 
  liable 
  to 
  be 
  

   broken 
  away 
  in 
  extricating 
  the 
  fossil 
  ; 
  and 
  then 
  there 
  remains 
  only 
  

   a 
  curious 
  bone 
  forked 
  from 
  the 
  humeral 
  articular 
  surface, 
  which 
  

   closely 
  resembles 
  the 
  precoraco-coracoid 
  portion 
  of 
  the 
  chelonian 
  

   pectoral 
  arch 
  just 
  described, 
  and 
  in 
  which 
  only 
  the 
  precoracoid 
  parts 
  

   similarly 
  meet 
  each 
  other 
  in 
  the 
  median 
  line. 
  A 
  beautiful 
  example 
  

   of 
  such 
  a 
  specimen 
  from 
  the 
  Kimmeridge 
  Clay, 
  presented 
  by 
  J. 
  C. 
  

   Mansell-Pleydell, 
  Esq., 
  is 
  exhibited 
  in 
  the 
  British 
  Museum 
  (fig. 
  2)*. 
  

  

  Fig. 
  2. 
  — 
  Coracoids 
  of 
  Hurgenosanrus. 
  

  

  Showing 
  connation 
  of 
  coracoid 
  (c) 
  and 
  precoracoid 
  (pc). 
  

  

  Such 
  a 
  specimen 
  goes 
  far 
  to 
  justify 
  an 
  interpretation 
  of 
  the 
  Plesio- 
  

   saurian 
  coracoids 
  as 
  the 
  combined 
  coracoid 
  and 
  precoracoid 
  bones. 
  

  

  The 
  Chelonian, 
  however, 
  has 
  the 
  form 
  of 
  its 
  pectoral 
  and 
  pelvic 
  

   arches 
  modified 
  by 
  the 
  enormous 
  amount 
  of 
  potential 
  epiphysial 
  

   ossification 
  which 
  characterizes 
  the 
  subclass. 
  The 
  epipleural 
  carti- 
  

   lages 
  of 
  the 
  Crocodile 
  and 
  the 
  small 
  epipleural 
  bones 
  of 
  Hatteria 
  

   and 
  of 
  Birds 
  are, 
  among 
  Chelonians, 
  changed 
  by 
  potential 
  growth 
  

   into 
  ossifications, 
  in 
  comparison 
  with 
  which 
  the 
  original 
  pleural 
  ele- 
  

   ments 
  are 
  small 
  ; 
  while 
  they 
  so 
  grow 
  and 
  unite 
  that 
  an 
  epipleural 
  

   skeleton 
  is 
  formed, 
  encased 
  not 
  in 
  muscles 
  as 
  in 
  most 
  other 
  ani- 
  

   mals, 
  but 
  in 
  representatives 
  of 
  muscles, 
  now 
  converted, 
  in 
  most 
  

   Chelonians, 
  into 
  horny 
  scutes 
  corresponding 
  with 
  muscles 
  in 
  their 
  

   extension. 
  

  

  * 
  It 
  is 
  at 
  present 
  standing 
  over 
  the 
  remains 
  of 
  Megalosaurus, 
  without 
  osteo- 
  

   logical 
  determination, 
  and 
  is 
  placed 
  upside 
  down. 
  

  

  