﻿AND 
  AVES 
  OF 
  NORTH 
  AMERICA. 
  33 
  

  

  Ischium 
  long 
  longitudinal, 
  posterior, 
  supporting 
  the 
  pubis 
  in 
  front 
  on 
  a 
  process. 
  

  

  Bibs 
  free, 
  double 
  headed. 
  

  

  Neural 
  arches 
  united 
  by 
  suture 
  ; 
  chevron 
  bones 
  present. 
  

  

  DINOSAURIA. 
  

  

  Limbs 
  ambulatory. 
  

  

  External 
  nostrils 
  anterior. 
  

  

  Inferior 
  pelvic 
  elements 
  in 
  contact 
  transversely, 
  acetabulum 
  imperforate. 
  

  

  Sacrum 
  of 
  six 
  vertebra?. 
  

  

  Neural 
  arches 
  attached 
  by 
  suture. 
  

  

  Premaxillary 
  single 
  or 
  double 
  ; 
  

  

  Teeth 
  wanting 
  or 
  represented 
  by 
  a 
  pair 
  of 
  tusks, 
  or 
  canines. 
  

  

  No 
  columella. 
  anomodontia. 
  

  

  Limbs 
  ambulatory. 
  

  

  External 
  nostrils 
  anterior. 
  

  

  Inferior 
  pelvic 
  elements 
  in 
  contact 
  transversely. 
  

  

  Sacrum 
  of 
  two 
  vertebra?. 
  

  

  A 
  columella. 
  

  

  Clavicle, 
  episternum 
  and 
  xiphisternum 
  present, 
  united. 
  

  

  Chevron 
  bones. 
  rhynchocephalia. 
  

  

  The 
  important 
  modification 
  in 
  the 
  mode 
  of 
  articulation 
  of 
  the 
  quadrate 
  bone, 
  which 
  

   characterizes 
  this 
  order 
  has 
  been 
  overlooked 
  in 
  most 
  of 
  the 
  systematic 
  arrangements 
  of 
  

   the 
  extinct 
  and 
  living 
  Reptilia. 
  The 
  subordinate 
  forms 
  differ 
  in 
  important 
  points, 
  but 
  the 
  

   groups 
  Sauropterygia, 
  Thecodontia, 
  and 
  Crocodilia, 
  appear 
  to 
  be 
  related 
  by 
  a 
  close 
  bond, 
  

   as 
  for 
  example 
  the 
  marine, 
  the 
  terrestrial, 
  the 
  Sphargicl, 
  and 
  the 
  Pleurodire 
  tortoises. 
  The 
  

   extremities 
  are 
  modified 
  for 
  all 
  modes 
  of 
  progression, 
  except 
  that 
  of 
  flight, 
  in 
  both 
  ; 
  while 
  

   as 
  much 
  gradation 
  between 
  these 
  types 
  is 
  seen 
  in 
  one 
  as 
  in 
  the 
  other. 
  In 
  the 
  characters 
  

   of 
  the 
  anterior 
  and 
  posterior 
  nasal 
  openings, 
  there 
  is 
  a 
  great 
  range 
  in 
  these 
  types, 
  but 
  

   the 
  transitions 
  in 
  these 
  respects 
  occur 
  successively 
  from 
  Crocodilus 
  to 
  Teleosaurus,* 
  to 
  

   Belodon, 
  to 
  Plesiosaurus 
  and 
  Nothosaurus. 
  

  

  An 
  important 
  definitive 
  character 
  is 
  found 
  in 
  several 
  types 
  of 
  the 
  Archosauria. 
  The 
  

   pterygoid 
  bones 
  are 
  prolonged 
  anteriorly 
  between 
  the 
  palatines, 
  and 
  frequently 
  as 
  far 
  as 
  

   the 
  vomer, 
  completely 
  separating 
  the 
  palatines. 
  The 
  latter 
  then 
  lie 
  exterior 
  to 
  the 
  

   pterygoids 
  and 
  between 
  them 
  and 
  the 
  maxillaries. 
  When 
  they 
  bear 
  teeth 
  the 
  latter 
  

   form 
  a 
  series 
  within 
  and 
  parallel 
  to 
  that 
  of 
  the 
  maxillary 
  bone. 
  

  

  This 
  structure 
  occurs 
  in 
  Sauropterygia, 
  as 
  Nothosaurus, 
  (see 
  fig. 
  4,) 
  and 
  in 
  Rhynchoce- 
  

  

  *See 
  Huxley 
  on 
  relations 
  of 
  Plesiosaurus 
  to 
  Teleosaurus, 
  Journ. 
  Geol. 
  Soc. 
  Loud. 
  

   AMERI. 
  PIIILOSO. 
  SOC. 
  — 
  VOL. 
  XIV. 
  9 
  

  

  