﻿AND 
  AVES 
  OF 
  NORTH 
  AMERICA. 
  183 
  

  

  MOSASAURID.E. 
  

  

  Dentition. 
  In 
  this 
  family 
  the 
  forms 
  of 
  the 
  teeth 
  differ 
  in 
  different 
  parts 
  of 
  the 
  maxil- 
  

   lary 
  bones. 
  Those 
  on 
  the 
  premaxillaries 
  are 
  more 
  cylindric 
  in 
  section, 
  and 
  the 
  compres- 
  

   sion, 
  or 
  development 
  of 
  angles 
  or 
  facets, 
  ' 
  whichever 
  it 
  be, 
  increases 
  as 
  they 
  succeed 
  each 
  

   other 
  posteriorly. 
  One 
  peculiarity 
  characterizes 
  the 
  teeth 
  of 
  the 
  most 
  of 
  the 
  true 
  Mosa- 
  

   sauri, 
  viz 
  : 
  the 
  anterior 
  aspect 
  bears 
  a 
  cutting 
  edge 
  more 
  or 
  less 
  developed, 
  while 
  in 
  Lio- 
  

   don 
  the 
  posterior 
  is 
  more 
  strongly 
  developed. 
  

  

  Os 
  Quadratum, 
  The 
  superior 
  extremity 
  of 
  the 
  os 
  quadratum 
  appears 
  to 
  have 
  had 
  

   considerable 
  motion 
  on 
  the 
  opisthotic. 
  Its 
  extent 
  is 
  so 
  much 
  greater 
  than 
  that 
  of 
  the 
  co- 
  

   tyloid 
  or 
  glenoid 
  cavity 
  applied 
  to 
  it, 
  as 
  to 
  suggest 
  a 
  gliding 
  motion, 
  especially 
  as 
  it 
  con- 
  

   stitutes 
  an 
  extensive 
  arc, 
  possessing 
  grooves 
  of 
  attachment 
  for 
  articular 
  cartilage 
  through- 
  

   out 
  its 
  length. 
  This 
  arc 
  is, 
  however, 
  bent 
  or 
  curved 
  in 
  the 
  horizontal 
  plane, 
  which 
  would 
  

   result 
  in 
  a 
  twisting 
  of 
  the 
  os 
  quadratum 
  round 
  its 
  long 
  axis, 
  should 
  the 
  motion 
  I 
  suggest 
  

   have 
  taken 
  place. 
  Such 
  a 
  twist 
  would 
  throw 
  the 
  proximal 
  portion 
  of 
  the 
  ramus 
  of 
  the 
  

   jaw 
  outwards, 
  a 
  motion 
  quite 
  necessary 
  to 
  the 
  horizontal 
  flexure 
  of 
  the 
  ramus 
  at 
  the 
  

   splenial 
  articulation, 
  which 
  no 
  doubt 
  took 
  place 
  in 
  swallowing 
  any 
  large 
  object. 
  The 
  ex- 
  

   tent 
  of 
  this 
  outward 
  deflexion 
  of 
  the 
  articular, 
  coronoid, 
  etc., 
  portions 
  of 
  the 
  jaw, 
  was 
  

   measured 
  by 
  the 
  outward 
  concavity 
  of 
  the 
  proximal 
  end 
  of 
  the 
  quadratum. 
  Thus 
  this 
  is 
  

   least 
  in 
  M. 
  depressus, 
  and 
  greater 
  in 
  M. 
  dekayi 
  and 
  M. 
  maximus 
  (see 
  cuts 
  fig. 
  48) 
  ; 
  the 
  

   great 
  projection 
  of 
  the 
  external 
  angle 
  in 
  Liodon 
  validus, 
  gave 
  that 
  species 
  an 
  excessive 
  

   power 
  of 
  dislocation, 
  and 
  the 
  same 
  peculiarity 
  in 
  Clidastes 
  propython, 
  was 
  followed 
  by 
  

   the 
  same 
  effect. 
  

  

  As 
  the 
  development 
  of 
  processes 
  and 
  ridges 
  on 
  the 
  ossa 
  quadrata 
  differ 
  in 
  the 
  different 
  

   species, 
  they 
  may 
  be 
  named 
  as 
  follows 
  : 
  The 
  proximal 
  articular 
  surface 
  extends 
  over 
  the 
  

   external 
  angle, 
  and 
  over 
  the 
  upper 
  edge 
  of 
  the 
  ala 
  forming 
  the 
  alar 
  process 
  (see 
  cuts). 
  

   Below 
  the 
  meatus 
  and 
  knob, 
  on 
  the 
  postero-external 
  margin, 
  there 
  is 
  a 
  ridge 
  which 
  ter- 
  

   minates 
  in 
  a 
  process 
  in 
  some 
  species, 
  to 
  be 
  called 
  the 
  median 
  posterior 
  ridge. 
  In 
  some 
  a 
  

   ridge 
  rises 
  from 
  the 
  outer 
  angle 
  of 
  the 
  distal 
  articular 
  face, 
  extending 
  outside 
  the 
  ridge 
  

   just 
  mentioned, 
  towards 
  the 
  pit, 
  called 
  the 
  distal 
  external 
  longitudinal. 
  In 
  front 
  of 
  this, 
  

   on 
  the 
  outer 
  face 
  of 
  the 
  quadrate 
  behind 
  or 
  near 
  the 
  origin 
  of 
  the 
  ala, 
  may 
  be 
  a 
  ridge 
  

   called 
  the 
  external 
  ridge. 
  See 
  Tab. 
  XI, 
  figs. 
  6—7. 
  

  

  Vertebrae. 
  As 
  has 
  been 
  already 
  pointed 
  out 
  by 
  Cuvier, 
  the 
  vertebra? 
  in 
  this 
  genus 
  tail 
  

   into 
  cervical, 
  dorsal, 
  sacro-lumbar 
  and 
  caudal 
  series. 
  The 
  cervicals 
  are 
  either 
  round 
  or 
  

   depressed 
  in 
  this 
  genus 
  ; 
  in 
  Liodon 
  the 
  articular 
  faces 
  are 
  either 
  round 
  or 
  compressed 
  in 
  

   the 
  species 
  I 
  have 
  examined. 
  They 
  are 
  arbitrarily 
  characterized 
  by 
  the 
  presence 
  of 
  an 
  

   obtuse 
  hypapophysis 
  which 
  has 
  an 
  articular 
  surface 
  for 
  a 
  separate 
  continuation 
  of 
  the 
  

   same, 
  as 
  in 
  Clidastes. 
  None 
  of 
  these 
  pieces 
  have 
  come 
  under 
  my 
  observation 
  in 
  this 
  

  

  