﻿396 
  PROCEEDmCS 
  of 
  the 
  national 
  museum 
  vol. 
  98 
  

  

  between 
  mandibles, 
  but 
  apparently 
  growth 
  of 
  hyoid 
  in 
  a 
  dorsoventral 
  

   direction 
  is 
  not 
  seriously 
  impeded 
  and 
  may 
  continue 
  indefinitely. 
  

   Individual 
  peculiarities 
  of 
  pitch, 
  range, 
  timber, 
  and 
  volume 
  of 
  voice 
  

   of 
  any 
  one 
  howler 
  is 
  reflected 
  by 
  corresponding 
  peculiarities 
  in 
  struc- 
  

   ture 
  of 
  its 
  hyoid. 
  Most 
  differentiated 
  portions 
  of 
  hyoid 
  of 
  seniculus 
  

   are 
  tentorium, 
  outline 
  and 
  diameters 
  of 
  opening 
  or 
  mouth, 
  the 
  area 
  

   immediately 
  bordering 
  it, 
  and 
  internal 
  wall 
  of 
  bulla. 
  In 
  males, 
  

   outline 
  of 
  lamina 
  of 
  tentorium 
  between 
  articular 
  depressions 
  sub- 
  

   circular, 
  oval, 
  crescentic, 
  rectangular, 
  square, 
  triangular, 
  or 
  trape- 
  

   zoidal; 
  surface 
  of 
  lamina 
  convex, 
  plane, 
  or 
  slightly 
  concave, 
  sometimes 
  

   bilobed; 
  lower 
  border 
  of 
  lamina 
  plane, 
  curved 
  inward, 
  or 
  slightly 
  

   outward; 
  outline 
  of 
  edge 
  of 
  lamina 
  even, 
  scalloped, 
  or 
  with 
  a 
  slight 
  

   median 
  incision 
  or 
  projection; 
  dorsal 
  surface 
  of 
  tentorium 
  rounded, 
  

   flat 
  or 
  angular. 
  Lateral 
  borders 
  of 
  hyoid 
  from 
  articular 
  depressions 
  

   of 
  tentorium 
  to 
  cornicules 
  may 
  be 
  inflated 
  or 
  compressed, 
  rounded, 
  

   flat, 
  or 
  angular 
  and 
  with 
  or 
  without 
  ridges 
  and 
  furrows; 
  cornicules 
  

   moderately 
  developed 
  or 
  obsolete, 
  their 
  bases 
  strongly 
  inflated 
  or 
  

   slightly 
  burled 
  ; 
  inner 
  surface 
  of 
  bulla 
  crossed 
  by 
  numerous 
  anastomo- 
  

   sing 
  ridges 
  and 
  trabeculae 
  or 
  traversed 
  by 
  only 
  a 
  short 
  midventral 
  

   line; 
  inner 
  lateral 
  bony 
  partitions 
  of 
  tentorium 
  variable 
  in 
  develop- 
  

   ment. 
  It 
  may 
  be 
  said, 
  in 
  short, 
  that 
  the 
  male 
  hyoid 
  is 
  a 
  considerably 
  

   more 
  flexible 
  structure 
  than 
  has 
  been 
  supposed 
  and 
  its 
  development 
  is 
  

   influenced 
  much 
  more 
  directly 
  by 
  exercise 
  of 
  muscles 
  and 
  chords 
  

   attached 
  to 
  it 
  than 
  by 
  the 
  genetic 
  constitution 
  of 
  the 
  individual. 
  

  

  Female 
  hyoids 
  are 
  much 
  less 
  specialized 
  and 
  correspondingly 
  less 
  

   variable 
  in 
  size 
  and 
  structure. 
  Hyoid 
  bones 
  of 
  very 
  young 
  individuals 
  

   lack 
  tentoria, 
  and 
  hyoids 
  of 
  half-grown 
  males 
  resemble 
  those 
  of 
  adult 
  

   females. 
  Additional 
  material 
  may 
  prove 
  that 
  characters 
  of 
  the 
  female 
  

   hyoid 
  are 
  much 
  more 
  reliable 
  for 
  determining 
  real 
  differences 
  and 
  

   relationships 
  between 
  species 
  of 
  howlers. 
  

  

  Hyoids 
  of 
  red 
  howlers 
  collected 
  in 
  various 
  localities 
  of 
  northern 
  

   Colombia 
  vary 
  in 
  the 
  same 
  order 
  individually 
  and 
  geographically. 
  

   External 
  and 
  cranial 
  variations 
  of 
  these 
  same 
  howlers 
  also 
  reveal 
  no 
  

   constant 
  local 
  differences. 
  

  

  ALOUATTA 
  PALLIATA 
  

  

  Plate 
  17, 
  d; 
  Figures 
  56, 
  57 
  

  

  Description. 
  — 
  Hyoid 
  bone 
  of 
  adult 
  male 
  A. 
  palliata 
  is 
  even 
  smaller 
  

   than 
  that 
  of 
  female 
  A. 
  seniculus. 
  Greatest 
  width 
  and 
  depth 
  of 
  

   smallest 
  and 
  largest 
  hyoid 
  specimens 
  at 
  hand, 
  both 
  of 
  fully 
  adult 
  

   males, 
  are 
  22 
  by 
  33 
  and 
  27 
  by 
  38 
  mm., 
  respectively. 
  Entire 
  posterior 
  

   face 
  of 
  apparatus 
  open 
  with 
  diameter 
  of 
  flaring 
  mouth 
  exceeding 
  depth 
  

   of 
  shallow 
  conelike 
  bulla. 
  Feature 
  corresponding 
  to 
  tentorium 
  in 
  

   seniculus 
  is 
  here 
  simply 
  a 
  broad 
  extension 
  or 
  lam.ina, 
  without 
  infolding, 
  

  

  