﻿434 
  PROCEEDINGS 
  OF 
  THE 
  NATIONAL 
  MUSEUM 
  vol. 
  io3 
  

  

  is 
  present 
  in 
  fair 
  abundance. 
  In 
  view 
  of 
  the 
  relative 
  scarcity 
  of 
  

   Hypertragulus, 
  Hypisodus, 
  and 
  Heteromeryx 
  in 
  the 
  Middle 
  Oligocene 
  

   compared 
  to 
  Leptomeryx 
  and 
  in 
  view 
  of 
  the 
  Late 
  Tertiary 
  development 
  

   of 
  this 
  suborder, 
  one 
  finds 
  very 
  attractive 
  the 
  suggestion 
  that 
  the 
  

   center 
  of 
  development 
  and 
  dispersal 
  was 
  more 
  southern 
  and 
  central 
  

   and 
  that 
  the 
  fu'st 
  three 
  genera 
  were 
  very 
  near 
  the 
  northern 
  limit 
  of 
  

   their 
  range. 
  

  

  That 
  none 
  of 
  the 
  genera 
  of 
  the 
  Plains 
  Province 
  invaded 
  the 
  Moun- 
  

   tain 
  Province 
  between 
  the 
  Lower 
  and 
  Middle 
  Oligocene 
  and 
  the 
  rather 
  

   long 
  list 
  of 
  genera 
  common 
  to 
  the 
  two 
  Provinces 
  during 
  both 
  ages 
  

   indicate 
  that 
  the 
  indigenous 
  genera 
  had 
  become 
  adjusted 
  to 
  their 
  

   environment, 
  or 
  ecologically 
  stabilized, 
  by 
  the 
  beginning 
  of 
  the 
  

   Oligocene 
  and 
  that 
  there 
  were 
  no 
  major 
  climatic 
  changes 
  before 
  the 
  

   close 
  of 
  the 
  Aliddle 
  Oligocene. 
  

  

  The 
  Eurasian 
  immigrants 
  embrace 
  six 
  subfamilies. 
  One, 
  the 
  Am- 
  

   phycynodontinae, 
  quicldy 
  became 
  adjusted 
  to 
  both 
  Provinces; 
  two, 
  

   the 
  Nimravinae 
  and 
  Anthracotherinae, 
  were 
  restricted 
  to 
  the 
  plains; 
  

   one, 
  the 
  Mustelinae, 
  was 
  divided 
  between 
  the 
  two 
  areas; 
  and 
  two, 
  the 
  

   Soricinae 
  and 
  Ochotoninae, 
  appeared 
  to 
  require 
  a 
  period 
  of 
  readjustment 
  

   before 
  invading 
  the 
  plains. 
  The 
  apparent 
  absence 
  of 
  Domnina 
  in 
  the 
  

   Lower 
  Oligocene 
  of 
  the 
  plains 
  is 
  certainly 
  open 
  to 
  question 
  since 
  its 
  

   very 
  small 
  size 
  greatly 
  reduces 
  its 
  chances 
  of 
  discovery. 
  On 
  the 
  other 
  

   hand, 
  the 
  zonal 
  distribution 
  of 
  mammals 
  is 
  more 
  clearly 
  reflected 
  by 
  

   the 
  small 
  species, 
  which 
  is 
  possibly 
  a 
  result 
  of 
  the 
  much 
  smaller 
  terri- 
  

   torial 
  range 
  of 
  the 
  individual. 
  However, 
  Desmatolagus 
  would 
  have 
  

   had 
  as 
  good 
  a 
  chance 
  of 
  being 
  preserved 
  as 
  Megalagus 
  or 
  Paleolagus 
  

   if 
  it 
  had 
  been 
  present 
  on 
  the 
  plains 
  during 
  the 
  Chadronian. 
  

  

  Ischyromys 
  is 
  the 
  same 
  size 
  as 
  Titanotheriomys, 
  and 
  had 
  it 
  been 
  on 
  

   the 
  plains 
  during 
  the 
  Chadronian 
  it 
  would 
  have 
  had 
  as 
  good 
  a 
  chance 
  of 
  

   being 
  preserved 
  as 
  the 
  latter. 
  The 
  occurrence 
  of 
  Ischyromys 
  in 
  the 
  

   Cypress 
  Hills 
  does 
  not 
  necessarily 
  indicate 
  that 
  the 
  fauna 
  was 
  mixed 
  

   (Wood, 
  1937, 
  p. 
  193), 
  asLambe(1908,p.8) 
  supposed, 
  but 
  could 
  indicate 
  

   temperature 
  zoning 
  on 
  the 
  plains 
  during 
  Lower 
  Oligocene 
  time. 
  The 
  

   large 
  ground 
  squirrels 
  of 
  the 
  genus 
  Citellus 
  (sensu 
  lato) 
  are, 
  with 
  very 
  

   few 
  exceptions, 
  restricted 
  to 
  the 
  mountains 
  and 
  the 
  Boreal 
  Zone 
  of 
  the 
  

   plains. 
  The 
  Cypress 
  HUls 
  are 
  far 
  enough 
  north 
  to 
  be 
  climatically 
  

   equal 
  to 
  Pipestone 
  Springs 
  regardless 
  of 
  what 
  was 
  the 
  Lower 
  Oligocene 
  

   climate 
  as 
  a 
  whole. 
  

  

  The 
  following 
  genera 
  have 
  been 
  omitted 
  from 
  table 
  4 
  because 
  our 
  

   knowledge 
  of 
  them 
  in 
  the 
  Lower 
  Oligocene 
  is 
  inadequate 
  for 
  this 
  type 
  of 
  

   study: 
  Talpaf, 
  Kentrogomphios, 
  Sinclairella, 
  Manitsha, 
  Ardynomys, 
  

   MacroUirsius 
  , 
  Hyaenodon, 
  Pseudopterodon, 
  Aepinacodon, 
  Bathygenys, 
  

   lAmnetes, 
  Heteromeryx, 
  Pseudoprotoceras, 
  Eotylopus. 
  

  

  