﻿AND 
  AVES 
  OF 
  NORTH 
  AMERICA. 
  177 
  

  

  walls, 
  composed 
  of 
  the 
  prolonged 
  prootic 
  in 
  front, 
  and 
  the 
  exoccipital 
  behind, 
  which 
  em- 
  

   braces 
  the 
  suspensorinm 
  for 
  much 
  of 
  its 
  length. 
  

  

  Eighteenth. 
  The 
  anterior 
  limbs 
  are 
  fins, 
  Avith 
  all 
  the 
  elements 
  in 
  a 
  single 
  plane 
  ; 
  the 
  

   radius 
  incapable 
  of 
  rotation 
  ; 
  the 
  humerus 
  broad 
  and 
  flat. 
  

  

  Nineteenth. 
  There 
  are 
  probably 
  no 
  hind 
  limbs. 
  

  

  Of 
  the 
  above 
  characters 
  the 
  first 
  eight 
  are 
  those 
  of 
  serpents 
  ; 
  the 
  five 
  characters 
  follow- 
  

   ing 
  the 
  ninth 
  are 
  lacertian, 
  while 
  the 
  seventeenth 
  is 
  peculiar, 
  and 
  not 
  found 
  in 
  any 
  

   existing 
  order 
  of 
  reptiles. 
  The 
  eighteenth 
  is 
  characteristic 
  of 
  the 
  Sauropterygia. 
  

  

  The 
  characters 
  of 
  the 
  teeth 
  are 
  much 
  those 
  of 
  serpents, 
  and 
  much 
  more 
  different 
  from 
  

   those 
  of 
  any 
  Saurians. 
  As 
  without 
  true 
  dentinal 
  fangs, 
  they 
  are 
  serpent-like 
  ; 
  for 
  the 
  ossi- 
  

   fication 
  of 
  the 
  pulp, 
  which 
  produces 
  a 
  fang-like 
  support 
  to 
  the 
  crown, 
  is 
  not 
  more 
  than 
  a 
  

   subordinate 
  character, 
  like 
  that 
  of 
  ossification 
  or 
  non-ossification 
  of 
  cartilages 
  in 
  many 
  

   existing 
  orders. 
  The 
  pterygoids, 
  which 
  are 
  in 
  contact 
  medially 
  in 
  Mosasaurus, 
  are 
  largely 
  

   free 
  in 
  Liodon 
  and 
  in 
  Clidastes 
  ; 
  in 
  the 
  latter 
  they 
  bear 
  teeth 
  as 
  abundantly 
  as 
  do 
  

   many 
  serpents. 
  Among 
  the 
  Lacertilia 
  the 
  dentition 
  is 
  either 
  truly 
  rhizodont 
  (the 
  Acro- 
  

   donta) 
  or 
  Pleurodont. 
  The 
  teeth 
  of 
  the 
  Varanidae 
  are 
  especially 
  different 
  from 
  those 
  of 
  

   the 
  present 
  order, 
  and 
  present 
  only 
  a 
  modification 
  of 
  the 
  pleurodont, 
  character. 
  The 
  outer 
  

   parapet 
  of 
  the 
  jaw 
  is 
  low, 
  and 
  the 
  shanks 
  proportionately 
  short 
  ; 
  they 
  are 
  in 
  addition 
  more 
  

   expanded 
  than 
  in 
  most 
  other 
  pleurodont 
  families. 
  

  

  The 
  characters 
  preserved 
  by 
  the 
  temporal 
  region 
  are 
  highly 
  peculiar 
  and 
  important 
  in 
  

   determining 
  the 
  affinities 
  of 
  the 
  group. 
  The 
  discovery 
  of 
  its 
  structure 
  furnishes 
  the 
  de- 
  

   sired 
  explanation 
  of 
  sundry 
  enigmatical 
  bones 
  which 
  occur 
  not 
  unfrequently 
  in 
  our 
  Creta- 
  

   ceous 
  formations. 
  In 
  the 
  following 
  diagnosis 
  the 
  present 
  is 
  compared 
  with 
  the 
  three 
  orders 
  

   to 
  which 
  it 
  makes 
  nearest 
  approach. 
  

  

  Testudinata. 
  

  

  Opisthotic 
  distinct, 
  closely 
  united 
  with 
  exoccipital 
  squamosal 
  and 
  prootic, 
  and 
  sup- 
  

   porting 
  squamosal 
  and 
  quadratum. 
  

  

  Lacertilia. 
  

  

  Opisthotic 
  distinct, 
  closely 
  attached 
  to 
  parietal 
  arc, 
  and 
  at 
  extremity 
  to 
  exoccipital 
  

   and 
  prootic 
  ; 
  supporting 
  squamosal 
  and 
  quadratum. 
  

  

  Pytlionomorpha. 
  

  

  Opisthotic 
  distinct, 
  not 
  or 
  scarcely 
  in 
  contact 
  with 
  parietal 
  arc 
  ; 
  embraced 
  at 
  one 
  end 
  

   by 
  prootic 
  and 
  exoccipital, 
  and 
  supporting 
  squamosal 
  and 
  quadratum. 
  

  

  Ophidia. 
  

  

  Opisthotic 
  distinct, 
  attached 
  only 
  to 
  prootic, 
  and 
  supporting 
  only 
  quadratum. 
  

  

  There 
  can 
  be 
  no 
  doubt 
  that 
  the 
  suspensorinm 
  of 
  Mosasaurus 
  is 
  homologous 
  with 
  the 
  

   element 
  in 
  the 
  tortoises 
  called 
  by 
  Huxley 
  opisthotic. 
  It 
  appears, 
  also, 
  to 
  be 
  homologous 
  

  

  AMERICA. 
  PHILO. 
  SOC 
  VOL. 
  XIV. 
  45. 
  

  

  