﻿392 
  E. 
  L. 
  Troxell 
  — 
  A 
  Tiny 
  Oligocene 
  Artio 
  dactyl. 
  

  

  tragulines 
  — 
  this 
  relation 
  is 
  based 
  primarily 
  on 
  the 
  foot 
  

   structure 
  — 
  but 
  they 
  are 
  far 
  more 
  advanced 
  even 
  than 
  

   these 
  in 
  the 
  modernization 
  of 
  the 
  long-crowned 
  teeth, 
  

   the 
  premolar 
  reduction, 
  the 
  form 
  of 
  the 
  occipital 
  region 
  

   of 
  the 
  skull, 
  and 
  the 
  orbit 
  surrounded 
  by 
  a 
  thin 
  promi- 
  

   nent 
  ring. 
  They 
  resemble 
  in 
  many 
  respects 
  the 
  little 
  

   Recent 
  Abyssinian 
  deer 
  called 
  M 
  ado 
  qua, 
  about 
  the 
  size 
  

   of 
  a 
  hare. 
  

  

  Because 
  of 
  the 
  nature 
  of 
  the 
  incisor-like 
  form 
  of 
  the 
  

   lower 
  canine 
  and 
  the 
  first 
  premolars, 
  these 
  animals 
  have 
  

   been 
  compared 
  to 
  Stenomylus, 
  3 
  and 
  represented 
  as 
  the 
  

   only 
  offshoots 
  from 
  an 
  isolated 
  stem, 
  immigrants 
  from 
  

   a 
  northern 
  center 
  of 
  dispersal. 
  Stenomylus 
  is 
  limited 
  to 
  

   the 
  lower 
  part 
  of 
  the 
  Miocene, 
  while 
  Hypisodus 
  is 
  known 
  

   only 
  from 
  the 
  Middle 
  Oligocene, 
  commonly 
  called 
  the 
  

   Brule 
  clays 
  or 
  Oreodon 
  beds 
  in 
  Nebraska, 
  Wyoming, 
  and 
  

   South 
  Dakota, 
  and 
  the 
  Cedar 
  Creek 
  beds 
  in 
  northeastern 
  

   Colorado. 
  In 
  many 
  respects 
  Hypisodus 
  is 
  like 
  the 
  primi- 
  

   tive 
  camels, 
  it 
  is 
  true, 
  but 
  the 
  resemblance 
  is 
  chiefly 
  in 
  

   the 
  possession 
  of 
  archaic 
  characters 
  which 
  might 
  ally 
  it 
  

   to 
  any 
  of 
  several 
  types. 
  It 
  may 
  be 
  near 
  a 
  central 
  group 
  

   from 
  which 
  spring 
  the 
  Tragulidse 
  (Hypertragulidse 
  in 
  

   America) 
  on 
  the 
  one 
  hand, 
  and 
  the 
  Tylopodidse 
  (camels, 
  

   etc.) 
  on 
  the 
  other, 
  and 
  Professor 
  Scott 
  4 
  seems 
  to 
  favor 
  

   the 
  latter. 
  

  

  Personally, 
  I 
  have 
  been 
  impressed 
  from 
  the 
  beginning 
  

   of 
  my 
  study, 
  in 
  a 
  rather 
  superficial 
  way, 
  perhaps, 
  with 
  

   the 
  very 
  camel-like 
  form 
  and 
  arrangement 
  of 
  the 
  molars 
  : 
  

   in 
  their 
  much 
  greater 
  anteroposterior 
  than 
  transverse 
  

   diameter 
  ; 
  in 
  the 
  prominent 
  para- 
  and 
  metastyles, 
  espe- 
  

   cially 
  on 
  M 
  3 
  ; 
  in 
  the 
  lake 
  ; 
  in 
  the 
  widely 
  separated 
  and 
  

   angular 
  lobes 
  of 
  the 
  lower 
  molars 
  ; 
  and 
  in 
  the 
  remark- 
  

   able 
  hypsodonty 
  of 
  all 
  the 
  teeth. 
  It 
  is 
  evident, 
  however, 
  

   that 
  these 
  little 
  animals 
  are 
  slightly 
  off 
  the 
  main 
  line 
  of 
  

   camel 
  descent, 
  if 
  we 
  admit 
  that 
  Poebrotherium 
  is 
  truly 
  

   in 
  the 
  direct 
  line, 
  since 
  it 
  is 
  impossible 
  to 
  derive 
  the 
  latter 
  

   from 
  an 
  ancestor 
  more 
  modernized 
  in 
  skull 
  and 
  teeth. 
  

   Poebrotherium 
  has 
  forty-four 
  teeth, 
  none 
  of 
  which 
  are 
  

   other 
  than 
  short-crowned, 
  and 
  the 
  orbits 
  are 
  not 
  enclosed 
  

   by 
  a 
  bony 
  ring. 
  

  

  . 
  3 
  This 
  peculiar 
  relation 
  to 
  Stenomylus 
  might 
  be 
  again 
  pointed 
  out 
  in 
  the 
  

   case 
  of 
  the 
  early 
  camels, 
  in 
  which 
  even 
  Protylopus 
  and 
  Poebrotherium 
  (this 
  

   does 
  not 
  apply 
  to 
  P. 
  andersoni 
  with 
  the 
  large 
  canines) 
  have 
  the 
  lower 
  canine 
  

   and 
  first 
  premolar 
  scarcely 
  larger 
  than 
  the 
  incisors. 
  

  

  4 
  W. 
  B. 
  Scott, 
  A 
  History 
  of 
  Land 
  Mammals 
  in 
  the 
  Western 
  Hemisphere, 
  

   p. 
  409, 
  1913. 
  

  

  