﻿56 
  MB. 
  B. 
  LYDEKKEE 
  ON 
  THE 
  BEMAUT6 
  ABB 
  AFFINITIES 
  

  

  This 
  genus 
  forms 
  a 
  link 
  connecting 
  other 
  forms 
  with 
  Pliosau 
  r 
  rus 
  r 
  

   and 
  will 
  be 
  characterized 
  by 
  its 
  comparatively 
  short 
  neck, 
  large 
  

   head 
  and 
  carinated 
  teeth, 
  elongated 
  mandibular 
  symphysis, 
  short 
  

   cervical 
  vertebrae, 
  having 
  donble 
  costal 
  facets 
  and 
  flattened 
  terminal 
  

   faces, 
  with 
  complete 
  anchylosis, 
  in 
  the 
  adult, 
  of 
  the 
  arches 
  and 
  ribs 
  

   with 
  the 
  centra. 
  In 
  the 
  pectoral 
  girdle 
  there 
  is 
  a 
  small 
  omo- 
  

   sternum 
  ; 
  the 
  scapulas 
  have 
  a 
  flattened 
  and 
  large 
  ventral 
  surface, 
  

   the 
  dorsal 
  portion 
  being 
  smaller 
  than 
  in 
  TJiaumatosaurus 
  ; 
  and 
  the 
  

   eoracoids 
  have 
  no 
  distinct 
  median 
  production 
  in 
  advance 
  of 
  the 
  line 
  

   of 
  the 
  glenoid 
  cavity. 
  In 
  the 
  limbs 
  the 
  epipodials 
  are 
  less 
  elongated 
  

   than 
  in 
  the 
  last-named 
  genus. 
  Finally, 
  Pliosaurus 
  itself 
  is 
  dis- 
  

   tinguished 
  by 
  its 
  still 
  larger 
  head 
  and 
  teeth 
  ; 
  the 
  still 
  shorter 
  neck 
  r 
  

   in 
  which 
  the 
  vertebrae 
  are 
  less 
  elongated 
  and 
  more 
  flattened, 
  the 
  

   costal 
  facets 
  are 
  more 
  distinctly 
  double, 
  and 
  the 
  arches 
  and 
  ribs 
  unite 
  

   merely 
  by 
  synchondrosis 
  with 
  the 
  centra. 
  The 
  pectoral 
  girdle 
  is 
  of 
  

   the 
  same 
  general 
  type 
  ; 
  but 
  the 
  omosternum 
  may 
  have 
  been 
  totally 
  lost. 
  

   Both 
  genera 
  have 
  similarly 
  elongated 
  ischia 
  : 
  but 
  the 
  epipodials 
  of 
  

   Pliosaurus 
  have 
  become 
  so 
  shortened 
  as 
  to 
  lose 
  all 
  resemblance 
  to 
  

   " 
  long 
  bones."' 
  

  

  With 
  the 
  evidence 
  of 
  Peloneustes 
  before 
  us 
  we 
  now 
  have 
  an 
  

   almost 
  continuous 
  chain 
  connecting 
  the 
  genus 
  Plesiosaurus 
  with 
  

   Pliosaurus, 
  the 
  course 
  of 
  evolution 
  being 
  directed 
  towards 
  a 
  gradual 
  

   increase 
  in 
  the 
  size 
  of 
  the 
  head, 
  in 
  the 
  length 
  of 
  the 
  mandibular 
  

   symphysis, 
  and 
  the 
  size 
  and 
  specialization 
  of 
  the 
  teeth, 
  accompanied 
  

   by 
  a 
  shortening 
  of 
  the 
  neck, 
  which 
  is 
  accomplished 
  by 
  a 
  reduction 
  

   both 
  in 
  the 
  number 
  and 
  length 
  of 
  the 
  component 
  vertebras, 
  and 
  also 
  

   by 
  a 
  tendency 
  to 
  a 
  loose 
  attachment 
  between 
  the 
  centra, 
  arches, 
  

   and 
  cervical 
  ribs 
  of 
  the 
  vertebras, 
  and 
  a 
  reduction 
  in 
  the 
  relative 
  

   length 
  of 
  the 
  epipodial 
  bones 
  of 
  the 
  limbs. 
  That 
  Peloneustes 
  phUar- 
  

   chus 
  is 
  the 
  direct 
  ancestor 
  of 
  Pliosaurus 
  is, 
  however, 
  improbable, 
  see- 
  

   ing 
  that 
  the 
  latter 
  genus 
  is 
  already 
  represented 
  in 
  the 
  Oxford 
  Clay. 
  

   Finally, 
  while 
  Pliosaurus 
  forms 
  the 
  culmination 
  of 
  the 
  series 
  just 
  

   indicated, 
  the 
  genus 
  Polyptychodon 
  appears 
  to 
  have 
  been 
  the 
  latest 
  

   development 
  of 
  the 
  series 
  of 
  which 
  the 
  middle 
  term 
  is 
  represented 
  

   bv 
  Cimoliosaurus. 
  

  

  Y. 
  The 
  Aeeixities 
  or 
  G-eosaeees. 
  

  

  In 
  a 
  recent 
  number 
  of 
  the 
  ' 
  Geological 
  ILagazine 
  ' 
  * 
  I 
  have 
  shown 
  

   that 
  the 
  genus 
  Geosaurus, 
  from 
  the 
  Lower 
  Ximeridgian 
  of 
  Bavaria, 
  

   is 
  undoubtedly 
  a 
  Crocodilian 
  allied 
  to 
  3Ietrior7ii/iichus 
  ; 
  and 
  a 
  

   skeleton 
  of 
  the 
  latter 
  genus 
  from 
  the 
  Oxford 
  Clay 
  near 
  Peterbo- 
  

   rough, 
  lately 
  sent 
  by 
  lEr. 
  A. 
  X. 
  Leeds 
  to 
  the 
  Xatural 
  History 
  

   Aiuseuru, 
  has 
  enabled 
  me 
  to 
  make 
  a 
  closer 
  comparison 
  between 
  the 
  

   two, 
  of 
  which 
  a 
  summary 
  is 
  now 
  given. 
  First, 
  it 
  appears 
  from 
  

   several 
  skeletons 
  in 
  the 
  collection 
  of 
  ILr. 
  Leeds, 
  as 
  well 
  as 
  from 
  the 
  

   type 
  of 
  Geosaurus, 
  that 
  these 
  Crocodiles 
  have 
  no 
  dermal 
  scutes, 
  but 
  

   that 
  bony 
  plates 
  were 
  developed 
  in 
  the 
  sclerotic 
  of 
  the 
  eye. 
  The 
  

  

  * 
  Decade 
  iii. 
  vol. 
  v. 
  p. 
  452 
  (1883). 
  

  

  