﻿AND 
  AYES 
  OF 
  NORTH 
  AMERICA. 
  201 
  

  

  readily 
  identified, 
  and 
  its 
  author 
  has 
  given 
  the 
  name 
  the 
  position 
  of 
  a 
  synonyme. 
  Owens' 
  

   Liodon 
  presents 
  teeth 
  similar 
  to 
  those 
  of 
  M. 
  impar, 
  but 
  here 
  again 
  we 
  do 
  not 
  know 
  any 
  

   other 
  characters 
  of 
  the 
  genus. 
  

  

  The 
  characters 
  are 
  well 
  distinguished 
  from 
  those 
  of 
  Clidastes 
  on 
  the 
  one 
  hand, 
  and 
  

   Mosasaurus 
  on 
  the 
  other. 
  The 
  vertebras 
  are 
  those 
  of 
  the 
  latter 
  genus, 
  while 
  the 
  ptery- 
  

   goids 
  do 
  not 
  present 
  that 
  union 
  on 
  the 
  median 
  line 
  characteristic 
  of 
  the 
  same. 
  They 
  have 
  

   not 
  the 
  horizontal 
  expansion 
  seen 
  in 
  Clidastes. 
  Their 
  teeth 
  are 
  not 
  arranged 
  after 
  a 
  type 
  

   so 
  strongly 
  pleurodont 
  as 
  in 
  Platecarpus, 
  but 
  in 
  the 
  type 
  species 
  they 
  are 
  more 
  exposed 
  

   than 
  in 
  Mosasaurus 
  giganteus, 
  M. 
  missuriensis 
  and 
  M. 
  dekayi. 
  As 
  Leidy 
  describes 
  the 
  

   pterygoid 
  bone 
  of 
  Platecarpus 
  tympaniticus, 
  I 
  am 
  induced 
  to 
  suppose 
  that 
  they 
  are 
  united 
  

   on 
  the 
  median 
  line; 
  the 
  width 
  above 
  is 
  nearly 
  one-third 
  the 
  length, 
  and 
  a 
  shelf 
  "projects 
  

   on 
  the 
  inner 
  side 
  separating 
  the 
  upper 
  from 
  the 
  lower 
  surface." 
  This 
  is 
  as 
  in 
  Mosasaurus, 
  

   and 
  constitutes 
  probably, 
  a 
  point 
  of 
  union 
  of 
  the 
  two 
  bones. 
  

  

  This 
  separation 
  of 
  the 
  Pterygoids 
  is 
  characteristic 
  of 
  Lacertilia 
  and 
  Ophidia 
  generally, 
  

   while 
  the 
  continuous 
  relation 
  belongs 
  rather 
  to 
  Crocodilia, 
  Sauropterygia, 
  etc. 
  

  

  The 
  entire 
  distinctness 
  of 
  the 
  chevron 
  bones 
  of 
  the 
  caudal 
  vertebra? 
  is 
  remarkable, 
  as 
  

   differing 
  from 
  the 
  known 
  structure 
  of 
  M. 
  giganteus, 
  M. 
  missuriensis, 
  M. 
  dekayi, 
  and 
  Cli- 
  

   dastes 
  propython. 
  The 
  large 
  size 
  of 
  the 
  L. 
  proriger 
  renders 
  it 
  improbable 
  that 
  this 
  fea- 
  

   • 
  ture 
  is 
  to 
  be 
  ascribed 
  to 
  immaturity, 
  especially 
  as 
  the 
  two 
  species 
  last 
  named 
  as 
  differino- 
  

   in 
  that 
  respect, 
  are 
  relatively 
  quite 
  small. 
  A 
  species 
  from 
  the 
  rotten 
  limestone 
  of 
  Ala- 
  

   bama, 
  presents 
  a 
  similar 
  characteristic 
  of 
  the 
  caudal 
  vertebra?, 
  but 
  whether 
  its 
  pterygoids 
  

   be 
  those 
  of 
  Lioclon 
  or 
  Mosasaurus, 
  I 
  am 
  unable 
  to 
  ascertain. 
  

  

  Pterygoids 
  nine, 
  smaller, 
  cylinclric 
  ; 
  mandibulars 
  eleven, 
  much 
  larger 
  ; 
  anterior 
  teeth 
  

   convex 
  inwardly, 
  indistinctly 
  fluted; 
  premaxilla 
  with 
  two 
  teeth 
  on 
  each 
  side, 
  prolonged 
  

   into 
  a 
  cylindric 
  rostrum. 
  l. 
  proriger. 
  

  

  Pterygoids 
  eight, 
  unequal, 
  the 
  median 
  large, 
  almost 
  equal 
  to 
  the 
  mandibulars 
  ; 
  teeth 
  

   compressed, 
  with 
  two 
  edges, 
  inner 
  sides 
  little 
  convex 
  ; 
  no 
  flutes 
  or 
  marked 
  facets. 
  

  

  L. 
  MITCHILLII. 
  

  

  The 
  only 
  character 
  by 
  which 
  I 
  distinguish 
  the 
  L. 
  validus 
  from 
  Mosasaurus, 
  is 
  the 
  pe- 
  

   culiar 
  form 
  of 
  the 
  os 
  quadratum. 
  The 
  L. 
  la?vis 
  is 
  characterized 
  by 
  the 
  form 
  of 
  its 
  verte- 
  

   bra?, 
  which 
  differs 
  considerably 
  from 
  that 
  of 
  any 
  species 
  of 
  Mosasaurus. 
  They 
  may 
  be 
  

   compared 
  as 
  follows: 
  

  

  Cervical 
  cups 
  vertically 
  oval, 
  proximal 
  caudal 
  subhexagonal 
  ; 
  surfaces 
  striate; 
  size 
  

   large. 
  l. 
  proriger. 
  

  

  Articular 
  faces 
  cervicals 
  and 
  anterior 
  dorsals 
  round 
  ; 
  posterior 
  dorsal 
  depressed, 
  centra 
  

   short, 
  broad; 
  large. 
  l. 
  validus. 
  

  

  AMEBICA. 
  PHILO. 
  SOC. 
  — 
  VOL 
  XIV. 
  51 
  

  

  