﻿AND 
  AVES 
  OF 
  NORTH 
  AMERICA. 
  185 
  

  

  group, 
  and 
  indeed 
  in 
  any 
  group 
  where 
  they 
  are 
  characterized 
  by 
  such 
  simplicity 
  of 
  form, 
  

   renders 
  it 
  probable 
  that 
  there 
  will 
  be 
  found 
  a 
  complete 
  gradation 
  between 
  different 
  genera 
  

   in 
  this 
  respect. 
  

  

  I. 
  Cervical 
  vertebrae 
  with 
  separated 
  articulating 
  hypapophyses. 
  

  

  Pterygoid 
  bones 
  united 
  on 
  the 
  middle 
  line. 
  

  

  The 
  pterygoid 
  teeth 
  pleurodont. 
  platecarptjs. 
  

  

  The 
  pterygoid 
  teeth 
  in 
  alveoli. 
  mosasaurus. 
  

  

  Pterygoid 
  bones 
  vertical, 
  separated 
  throughout 
  their 
  length. 
  liodon. 
  

  

  II. 
  Cervical 
  vertebra? 
  with 
  simple 
  continuous 
  hypapophyses. 
  baptosaurus. 
  

  

  Two 
  genera 
  have 
  been 
  defined 
  from 
  dental 
  characters, 
  as 
  follows 
  : 
  

   Crowns 
  of 
  the 
  teeth 
  curved, 
  with 
  one 
  face 
  smaller 
  than 
  the 
  other, 
  more 
  or 
  less 
  atten- 
  

   uated 
  and 
  acute 
  ; 
  numerous 
  narrow 
  ridges 
  on 
  inner 
  face. 
  holcodus. 
  

  

  Crowns 
  of 
  the 
  teeth 
  straight, 
  compressed, 
  more 
  slender, 
  regularly 
  acuminate 
  ; 
  faces 
  

   equal, 
  facets 
  distinct, 
  numerous. 
  poltgonodon. 
  

  

  The 
  genus 
  Diplotomodon, 
  Leidy, 
  known 
  from 
  a 
  single 
  very 
  much 
  compressed 
  dental 
  

   crown 
  with 
  minutely 
  denticulated 
  edges, 
  may 
  belong 
  here. 
  

  

  To 
  what 
  extent 
  and 
  in 
  what 
  manner 
  the 
  dental 
  characters 
  of 
  those 
  genera 
  established 
  

   on 
  them, 
  coincide 
  with 
  those 
  distinguishing 
  the 
  genera 
  of 
  the 
  table, 
  it 
  is 
  not 
  now 
  easy 
  to 
  

   discover. 
  In 
  species 
  referable 
  to 
  Macmsawus, 
  teeth 
  of 
  the 
  compressed 
  form 
  and 
  lenticu- 
  

   lar 
  section 
  characteristic 
  of 
  Liodon, 
  are 
  found 
  in 
  one 
  species, 
  while 
  another 
  presents 
  the 
  

   more 
  angular 
  facetted 
  section 
  of 
  Mosasaurus 
  in 
  the 
  anterior 
  portion 
  of 
  the 
  series. 
  Both 
  

   forms 
  seem 
  to 
  occur 
  in 
  Mosasaurus 
  proper, 
  according 
  to 
  Leidy. 
  Of 
  these 
  genera 
  founded 
  

   on 
  dental 
  forms, 
  Polygonodon, 
  Leidy, 
  is 
  the 
  best 
  marked, 
  and 
  will 
  prove 
  distinct 
  from 
  any 
  

   of 
  the 
  above. 
  

  

  The 
  humerus 
  of 
  a 
  very 
  large 
  reptile 
  in 
  the 
  Mus. 
  Acad., 
  Phil., 
  has 
  been 
  regarded 
  by 
  

   Agassiz 
  as 
  belonging 
  to 
  a 
  Chelonioid, 
  and 
  by 
  Leidy 
  been 
  referred 
  to 
  the 
  common 
  Mosa- 
  

   saurus 
  of 
  New 
  Jersey. 
  There 
  is 
  at 
  present 
  no 
  means 
  of 
  assigning 
  it 
  to 
  any 
  species 
  of 
  this 
  

   order 
  otherwise 
  defined. 
  It 
  is 
  the 
  Atlantoclielys 
  mortoni 
  Agass., 
  and 
  may 
  refer 
  to 
  any 
  large 
  

   species 
  of 
  the 
  genus 
  so 
  far 
  as 
  our 
  knowledge 
  goes 
  ; 
  it 
  was 
  not 
  originally 
  described, 
  however, 
  

   and 
  cannot 
  therefore 
  retain 
  this 
  name. 
  The 
  Mosasaurus 
  deJcayi, 
  Bronn, 
  is 
  founded 
  on 
  a 
  

   tooth 
  exactly 
  like 
  that 
  of 
  If. 
  mitchillii 
  of 
  Leidy, 
  and 
  cannot 
  be 
  distinguished 
  on 
  such 
  basis 
  

   alone. 
  The 
  M. 
  impar 
  is 
  only 
  known 
  from 
  jaws 
  and 
  teeth, 
  and 
  hence 
  is 
  the 
  only 
  species 
  

   whose 
  name 
  is 
  liable 
  to 
  have 
  been 
  duplicated 
  here. 
  It 
  may 
  belong 
  to 
  any 
  of 
  the 
  Ameri- 
  

   can 
  species 
  of 
  Liodon 
  here 
  enumerated, 
  except 
  L. 
  va/ldus 
  and 
  L. 
  proriger,' 
  whose 
  

  

  AMERICA. 
  PHILO. 
  SOC. 
  — 
  VOL. 
  XIV. 
  47 
  

  

  