﻿184 
  PROCEEDINGS 
  OF 
  THE 
  NATIONAL 
  MUSEUM 
  vol. 
  103 
  

  

  the 
  ocean 
  must 
  have 
  lowered 
  the 
  sea 
  level 
  50 
  to 
  90 
  meters 
  below 
  the 
  

   present 
  level 
  (Daly, 
  1934, 
  pp. 
  41-50). 
  The 
  depth 
  of 
  the 
  eastern 
  part 
  

   of 
  the 
  Bering 
  Sea 
  (withm 
  the 
  peruneter 
  passing 
  through 
  the 
  eastern 
  

   projection 
  of 
  Siberia, 
  the 
  Seward 
  Peninsula 
  of 
  Alaska, 
  and 
  Nunivak 
  

   and 
  St. 
  Lawrence 
  Islands) 
  averages 
  less 
  than 
  40 
  meters. 
  Thus 
  the 
  

   ocean 
  bottom 
  between 
  Alaska 
  and 
  Asia 
  must 
  have 
  emerged 
  with 
  each 
  

   glacial 
  stage, 
  forming 
  a 
  wide 
  connection 
  between 
  the 
  two 
  continents. 
  

   This 
  process 
  may 
  have 
  been 
  enhanced 
  by 
  the 
  uplifting 
  of 
  the 
  area 
  

   along 
  the 
  margin 
  of 
  the 
  ice 
  cap, 
  brought 
  about 
  by 
  plastic 
  or 
  elastic 
  

   deformation 
  of 
  the 
  earth 
  crust 
  under 
  the 
  weight 
  of 
  the 
  ice 
  masses. 
  

  

  A 
  land 
  bridge 
  between 
  Alaska 
  and 
  Asia 
  must 
  have 
  persisted 
  for 
  

   prolonged 
  periods 
  of 
  time. 
  Simultaneously, 
  Alaska 
  was 
  discon- 
  

   nected 
  from 
  other 
  inhabitable 
  areas 
  of 
  North 
  America 
  by 
  broad 
  ice 
  

   fields. 
  

  

  In 
  view 
  of 
  the 
  geological 
  history 
  of 
  Alaska 
  we 
  may 
  conclude 
  that 
  

   the 
  present 
  Alaskan 
  fresh-water 
  triclads 
  are 
  the 
  remnants 
  or 
  successors 
  

   of 
  triclads 
  that 
  lived 
  in 
  Alaska 
  in 
  the 
  glacial 
  times 
  and 
  perhaps 
  even 
  

   in 
  preglacial 
  periods, 
  and 
  that 
  they 
  are, 
  in 
  all 
  probability, 
  of 
  Asiatic 
  

   origin. 
  It 
  may 
  further 
  be 
  assumed 
  that 
  some 
  Alaskan 
  triclads 
  have 
  

   extended 
  their 
  range 
  during 
  the 
  postglacial 
  period 
  and 
  proceeded 
  

   southeast 
  along 
  the 
  Rocky 
  Mountains.^ 
  The 
  occurrence 
  of 
  a 
  species 
  

   of 
  Polycelis 
  in 
  Wyoming 
  and 
  South 
  Dakota 
  may 
  have 
  resulted 
  from 
  

   this 
  migration. 
  More 
  light 
  would 
  be 
  thrown 
  on 
  this 
  question 
  by 
  a 
  

   more 
  thorough 
  study 
  of 
  the 
  triclad 
  fauna 
  of 
  the 
  Canadian 
  and 
  

   American 
  Rockies. 
  

  

  Literature 
  cited 
  

  

  Arndt, 
  Waltheb 
  

  

  1922. 
  Untersuchungen 
  an 
  Bachtricladen. 
  Ein 
  Beitrag 
  zur 
  Kenntnis 
  der 
  

  

  Paludicolen 
  Korsikas, 
  Rumaniens 
  und 
  Sibiriens. 
  Zeitschr. 
  fur 
  wiss. 
  

  

  Zool., 
  vol. 
  120, 
  pp. 
  98-146, 
  pi. 
  4. 
  

   Bazikalova, 
  a. 
  fA. 
  

  

  1947. 
  Turbellaria-Triclada 
  vostochnol 
  Sibiri 
  i 
  Pribalkal'fa. 
  Doklady 
  Akad. 
  

  

  Nauk 
  SSSR, 
  new 
  ser., 
  vol. 
  55, 
  pp. 
  671-672. 
  

   Beauchamp, 
  Paul 
  be 
  

  

  1932. 
  Biospeologica. 
  LVI. 
  Turbellaries, 
  Hirudinees, 
  Branchiobdellidfe. 
  

  

  Deuxifeme 
  s^rie. 
  Arch. 
  Zool. 
  Exp^r. 
  et 
  Gen., 
  vol. 
  73, 
  pp. 
  113-380, 
  

  

  pis. 
  6-8. 
  

   1939. 
  La 
  syst^matique 
  et 
  I'^thologie 
  des 
  Fonticola 
  (Turb. 
  Triclades). 
  Vgstnfk 
  

  

  Ceskoslov. 
  Zool. 
  Spolecnosti 
  v 
  Praze, 
  vol. 
  6-7, 
  pp. 
  91-96. 
  

   1947. 
  Observations 
  sur 
  quelques 
  Turbellaries 
  du 
  Mus6e 
  royal 
  d'Histoire 
  

  

  naturelle 
  de 
  Belgique. 
  Bull. 
  Mus. 
  Roy. 
  Hist. 
  Nat. 
  Belgique, 
  vol. 
  

  

  23, 
  No. 
  33, 
  11 
  pp. 
  

  

  1949. 
  Biospeologica. 
  LXIX. 
  Turbellaries 
  (troisifeme 
  s^rie). 
  Arch. 
  Zool. 
  

  

  Exp^r. 
  et 
  G6n., 
  vol. 
  86, 
  Notes 
  et 
  Revue, 
  pp. 
  50-65. 
  

  

  1950. 
  Nouvelles 
  diagnoses 
  de 
  triclades 
  obscuricoles. 
  Bull. 
  Soc. 
  Zool. 
  France, 
  

  

  vol. 
  75, 
  pp. 
  65-70. 
  

  

  • 
  I 
  wish 
  here 
  to 
  correct 
  my 
  assumption 
  in 
  a 
  previous 
  paper 
  (Kent, 
  1943, 
  p. 
  6) 
  that 
  all 
  Canadian 
  triclads 
  

   had 
  entered 
  Canada 
  from 
  the 
  south. 
  

  

  