﻿436 
  

  

  H. 
  G. 
  SEELEY 
  ON 
  SOME 
  GENERIC 
  MODIFICATIONS 
  

  

  34. 
  Note 
  on 
  some 
  of 
  the 
  Generic 
  Modifications 
  of 
  the 
  Plesio- 
  

   saurian 
  Pectoral 
  Arch. 
  By 
  Harry 
  Gr. 
  Seeley, 
  Esq., 
  P.L.S., 
  

   F.G.S. 
  (Read 
  May 
  13, 
  1874.) 
  

  

  In 
  not 
  having 
  a 
  sternum 
  the 
  Plesiosauria 
  differ 
  from 
  the 
  Croco- 
  

   dilia 
  and 
  from 
  all 
  the 
  Lacertian 
  orders 
  of 
  Reptiles. 
  Serpents 
  with 
  

   limbs 
  being 
  as 
  yet 
  undiscovered, 
  the 
  only 
  true 
  Eeptilia 
  which 
  

   admit 
  of 
  comparison 
  with 
  Plesiosaurs 
  in 
  the 
  pectoral 
  bones 
  are 
  the 
  

   Chelonians. 
  And 
  even 
  here, 
  at 
  first 
  sight, 
  the 
  resemblance 
  is 
  not 
  

   so 
  evident 
  as 
  to 
  command 
  attention 
  ; 
  for 
  the 
  shapes 
  of 
  the 
  plastron- 
  

   bones 
  in 
  embryonic 
  Tortoises 
  are 
  more 
  suggestive 
  of 
  the 
  pectoral 
  

   and 
  pelvic 
  girdles 
  of 
  Plesiosaurians 
  than 
  are 
  the 
  internal 
  chelonian 
  

   bones 
  which 
  support 
  the 
  limbs, 
  since 
  in 
  Plesiosaurs 
  these 
  osteological 
  

   elements 
  are 
  expanded 
  shields 
  which 
  cover 
  much 
  of 
  the 
  abdominal 
  

   surface. 
  When, 
  however, 
  the 
  embryonic 
  pectoral 
  arch 
  of 
  such 
  a 
  

   Chelonian 
  as 
  the 
  Ghelone 
  my 
  das* 
  (fig. 
  1) 
  is 
  critically 
  looked 
  at, 
  only 
  

  

  Pig. 
  1. 
  — 
  Pectoral 
  Arch 
  and 
  Bones 
  of 
  Plastron 
  of 
  Chelone 
  mydas. 
  

   (After 
  Parker.) 
  

  

  c. 
  Coracoid. 
  

  

  fc. 
  Precoracoid. 
  

  

  s. 
  Scapula. 
  

  

  unimportant 
  osteological 
  modification 
  is 
  needed 
  to 
  change 
  its 
  cha- 
  

   racters 
  to 
  those 
  of 
  a 
  Plesiosaur. 
  

  

  The 
  chelonian 
  coracoid 
  bones 
  (c) 
  are 
  rod-like 
  ; 
  but 
  their 
  extension 
  

   is 
  entirely 
  posterior 
  to 
  the 
  articulation 
  for 
  the 
  humerus 
  : 
  the 
  bones 
  

   approximate 
  somewhat 
  posteriorly, 
  are 
  somewhat 
  concave 
  on 
  their 
  

   outer 
  margin, 
  and 
  terminate 
  in 
  cartilages 
  of 
  a 
  shoe-shaped 
  form, 
  

   which 
  are 
  so 
  extended 
  inward 
  that 
  their 
  toe-like 
  terminations 
  meet 
  

   in 
  the 
  median 
  line. 
  Then, 
  from 
  the 
  humeral 
  articulation 
  the 
  two 
  

   precoracoids 
  (pc) 
  extend 
  inward 
  towards 
  the 
  median 
  line 
  ; 
  they 
  are 
  

   inclined 
  very 
  slightly 
  forward, 
  and 
  join 
  either 
  by 
  their 
  cartilages 
  or 
  

   intervening 
  connective 
  tissue. 
  

  

  If, 
  now, 
  a 
  line 
  be 
  drawn 
  to 
  join 
  the 
  median 
  points 
  of 
  meeting 
  of 
  

   * 
  Mr. 
  Parker, 
  "Shoulder-girdle" 
  (Ray 
  Society), 
  pi. 
  xii. 
  fig. 
  1. 
  

  

  