﻿442 
  H. 
  G. 
  SEELEY 
  ON 
  SOME 
  GENEBIC 
  MODIFICATIONS 
  

  

  the 
  affinities 
  of 
  Plesiosaurus, 
  by 
  which 
  alone 
  it 
  became 
  possible 
  to 
  

   regard 
  the 
  pectoral 
  as 
  the 
  pelvic 
  girdle. 
  I 
  allude 
  especially 
  to 
  the 
  

   fig. 
  116, 
  p. 
  310 
  (G-eology 
  of 
  Oxford), 
  which 
  represents, 
  I 
  think, 
  the 
  

   pectoral 
  arch 
  of 
  a 
  new 
  genus 
  with 
  the 
  posterior 
  end 
  turned 
  forwards. 
  

   The 
  forms 
  of 
  these 
  bones 
  are 
  exactly 
  paralleled 
  by 
  pectoral 
  bones 
  of 
  

   undescribed 
  genera 
  of 
  Plesiosauria 
  from 
  the 
  great 
  Pelolithic 
  Period 
  

   of 
  Oxford 
  to 
  Kimmeridge 
  Clay. 
  The 
  fact 
  that 
  no 
  ilium 
  was 
  found 
  

   by 
  Professor 
  Phillips 
  is 
  similarly 
  explained. 
  Diagram 
  180, 
  p. 
  379, 
  

   marked 
  " 
  ischium," 
  appears 
  to 
  me 
  to 
  be 
  a 
  left 
  scapula 
  ; 
  diagram 
  179, 
  

   p. 
  378, 
  named 
  " 
  coracoid 
  of 
  Plesiosaurus, 
  " 
  is 
  the 
  right 
  ischium 
  ; 
  dia- 
  

   gram 
  177, 
  called 
  " 
  ischial 
  bones 
  of 
  Plesiosaurus," 
  appears 
  to 
  me 
  to 
  

   represent 
  the 
  scapulas 
  of 
  a 
  new 
  genus. 
  

  

  The 
  English 
  Plesiosauria 
  hitherto 
  indicated 
  have 
  been 
  arranged 
  

   in 
  the 
  genera 
  Plesiosaurus, 
  Pliosaurus, 
  Polyptycliodon, 
  Stereosaurus 
  

   (which 
  is 
  a 
  genus 
  instituted 
  for 
  the 
  stiff-backed 
  Plesiosaurs 
  of 
  the 
  

   Cambridge 
  Greensand), 
  Placodus, 
  and 
  Tanystrophceus* 
  . 
  Very 
  little 
  is 
  

   known 
  as 
  yet 
  of 
  most 
  of 
  these, 
  especially 
  of 
  the 
  pectoral 
  arch 
  ; 
  so 
  that 
  

   I 
  prefer 
  to 
  limit 
  this 
  note 
  to 
  the 
  genus 
  Plesiosaurus 
  as 
  it 
  is 
  usually 
  

   understood. 
  

  

  Plesiosaurs 
  may 
  be 
  divided 
  into 
  those 
  which 
  are 
  furnished 
  with 
  a 
  

   separate 
  interclavicle 
  and 
  those 
  in 
  which 
  that 
  bone 
  has 
  no 
  separate 
  

   existence. 
  Of 
  the 
  first 
  family, 
  the 
  Plesiosauridse, 
  the 
  type 
  should 
  

   be 
  Plesiosaurus 
  dolichodeirus 
  of 
  Conybeare 
  ; 
  but 
  none 
  of 
  the 
  speci- 
  

   mens 
  so 
  named 
  in 
  the 
  British 
  Museum 
  gives 
  certain 
  evidence 
  about 
  

   its 
  pectoral 
  bones. 
  Of 
  the 
  pectoral 
  bones 
  of 
  other 
  Plesiosaurs, 
  Cony- 
  

   beare 
  gave, 
  in 
  1824, 
  a 
  restoration 
  which 
  has 
  not 
  since 
  been 
  mate- 
  

   rially 
  improved 
  upon 
  (fig. 
  6), 
  chiefly 
  made 
  from 
  specimens 
  said 
  to 
  be 
  

  

  Fig. 
  6. 
  — 
  Pectoral 
  Arch 
  of 
  Plesiosaurus, 
  restored 
  by 
  Conybeare. 
  

  

  st. 
  Sternum, 
  cl. 
  Clavicle, 
  s. 
  Scapula, 
  c. 
  Coracoid. 
  g. 
  Glenoid 
  cavity. 
  

  

  in 
  the 
  Oxford 
  Museum, 
  but 
  which 
  seem 
  to 
  have 
  long 
  been 
  mislaid. 
  

   Conybeare 
  correctly 
  identified 
  the 
  coracoids, 
  and 
  accurately 
  placed 
  

   the 
  interclavicle 
  (sternum) 
  anterior 
  to 
  them, 
  so 
  that 
  its 
  hinder 
  part 
  is 
  

   overlapped 
  and 
  hidden 
  by 
  the 
  coracoids. 
  The 
  scapula 
  correctly 
  formed 
  

   part 
  of 
  the 
  humeral 
  articular 
  surface 
  behind 
  ; 
  and 
  in 
  front 
  it 
  over- 
  

   lapped 
  a 
  lateral 
  wing 
  of 
  the 
  interclavicle. 
  The 
  scapula 
  in 
  this 
  spe- 
  

  

  * 
  The 
  last 
  two 
  genera 
  are 
  from 
  the 
  Bhaetic 
  beds, 
  — 
  Placodus 
  in 
  the 
  collection 
  of 
  

   Mr. 
  C. 
  Moore, 
  of 
  Bath, 
  Tanystrophceus 
  in 
  that 
  of 
  Mr. 
  James 
  Plant, 
  of 
  Leicester. 
  

  

  