﻿410 
  PROCEEDINGS 
  OF 
  THE 
  NATIONAL 
  MUSEUM 
  vol. 
  103 
  

  

  Upper 
  Lower 
  

  

  P4-M3 
  6.0 
  6.7 
  

  

  Length 
  Width 
  Length 
  Width 
  

  

  P4 
  1.6 
  1.7 
  1.7 
  1.5 
  

  

  M, 
  1.5 
  1.9 
  1.7 
  1.6 
  

  

  M2 
  1.5 
  1.9 
  1.6 
  1.7 
  

  

  M3 
  1.2 
  1.7 
  1.5 
  1.4 
  

  

  The 
  characters 
  which 
  distinguish 
  this 
  species 
  are 
  the 
  larger 
  size, 
  the 
  

   elongate 
  P4, 
  and 
  the 
  short, 
  nearly 
  quadrangular 
  M3, 
  

  

  Family 
  Cricetidae 
  

  

  This 
  family 
  of 
  rodents 
  is 
  represented 
  in 
  this 
  area 
  by 
  a 
  single 
  genus, 
  

   Eumys, 
  and 
  most 
  of 
  the 
  specimens 
  came 
  from 
  two 
  localities, 
  Nos. 
  

   24LC15 
  and 
  24LC17. 
  The 
  others, 
  specimens 
  in 
  the 
  Carnegie 
  Muse- 
  

   um, 
  are 
  from 
  the 
  upper 
  levels 
  of 
  No. 
  24BW18, 
  locally 
  known 
  as 
  the 
  

   Old 
  Hadcock 
  Ranch. 
  This 
  series 
  of 
  nearly 
  100 
  specimens, 
  although 
  

   showing 
  considerable 
  variation, 
  is 
  divisible 
  into 
  four 
  groups, 
  three 
  of 
  

   which 
  exhibit 
  characters 
  of 
  the 
  teeth 
  not 
  readily 
  referable 
  to 
  species 
  

   described 
  from 
  the 
  White 
  River 
  deposits 
  of 
  the 
  plains. 
  All 
  of 
  the 
  

   specimens 
  are 
  15 
  to 
  25 
  percent 
  larger 
  than 
  those 
  of 
  the 
  plains. 
  Also, 
  

   the 
  teeth 
  are 
  higher 
  crowned 
  and 
  the 
  cusps 
  are 
  better 
  developed. 
  

  

  By 
  analogy 
  with 
  some 
  of 
  the 
  wide-ranging 
  species 
  of 
  the 
  genus 
  

   Peromyscus, 
  one 
  would 
  normally 
  expect 
  only 
  subspecific 
  differences 
  

   between 
  the 
  members 
  of 
  the 
  genus 
  Eumys 
  from 
  the 
  Badlands 
  of 
  South 
  

   Dakota 
  and 
  the 
  Canyon 
  Ferry 
  Reservoir 
  area 
  since 
  the 
  former 
  is 
  on 
  

   the 
  plains 
  and 
  the 
  latter 
  is 
  in 
  the 
  mountains. 
  On 
  the 
  other 
  hand, 
  in 
  

   the 
  mountain 
  areas 
  two 
  or 
  three 
  distinct 
  species 
  of 
  Peromyscus 
  

   usually 
  occur 
  in 
  a 
  single 
  area 
  in 
  addition 
  to 
  the 
  plains 
  species. 
  This 
  

   appears 
  to 
  be 
  correlated 
  with 
  the 
  greater 
  variety 
  of 
  ecological 
  niches 
  

   within 
  a 
  limited 
  area, 
  and 
  the 
  remains 
  of 
  a 
  varied 
  fauna 
  from 
  a 
  

   variety 
  of 
  ecological 
  niches 
  could 
  be 
  concentrated 
  in 
  a 
  very 
  small 
  area 
  

   of 
  deposition 
  by 
  the 
  work 
  of 
  owls. 
  In 
  view 
  of 
  the 
  geologic 
  history 
  of 
  

   this 
  region, 
  the 
  environment 
  for 
  small 
  rodents 
  during 
  the 
  Oligocene 
  

   was 
  probably 
  as 
  varied 
  and 
  as 
  different 
  from 
  that 
  of 
  the 
  plains 
  as 
  it 
  

   is 
  today. 
  Although 
  only 
  one 
  of 
  the 
  plains 
  species 
  has 
  been 
  positively 
  

   identified 
  in 
  this 
  material, 
  A. 
  E. 
  Wood 
  (1937, 
  p. 
  250) 
  reports 
  E. 
  

   elegans 
  Leidy 
  from 
  Montana 
  but 
  gives 
  no 
  locality 
  data. 
  

  

  The 
  following 
  artificial 
  key 
  gives 
  the 
  distinguishing 
  characters 
  

   of 
  the 
  species 
  of 
  this 
  area: 
  

  

  Key 
  to 
  the 
  species 
  of 
  Eumys 
  

  

  a.* 
  Anteroconid 
  on 
  Mj 
  large, 
  anterior 
  cingulum 
  on 
  M2 
  extending 
  the 
  full 
  width 
  

   of 
  tooth, 
  a 
  short 
  but 
  distinct 
  mesolophid 
  on 
  M2 
  cricetodontoides 
  

  

  a} 
  Anteroconid 
  on 
  Mi 
  small, 
  mesolophid 
  on 
  M2 
  obsolete 
  or 
  absent. 
  

   hy 
  Cross 
  lophs 
  normal. 
  

  

  