﻿422 
  Wieland 
  — 
  Evolution 
  of 
  the 
  Testudinate 
  Humerus. 
  

  

  As 
  a 
  modification 
  of 
  mechanical 
  change 
  the 
  instance 
  of 
  

   responsive 
  variation 
  afforded 
  by 
  the 
  peculiar 
  curvature 
  of 
  the 
  

   humerus 
  to 
  conform 
  to 
  the 
  presence 
  of 
  the 
  carapace 
  and 
  plas- 
  

   tron 
  is 
  impressive. 
  The 
  external 
  armature 
  once 
  developed, 
  

   the 
  humerus 
  seems 
  to 
  have 
  continuously 
  accommodated 
  itself 
  

   to 
  it 
  during 
  all 
  the 
  secondary 
  changes 
  subsequently 
  undergone. 
  

  

  Much 
  interest 
  too 
  centers 
  in 
  the 
  transitional 
  forms. 
  That 
  

   the 
  gap 
  between 
  the 
  chelic 
  and 
  thalassic 
  humeri 
  must 
  neces- 
  

   sarily 
  have 
  been 
  bridged 
  over 
  by 
  primitive 
  straighter 
  shafts 
  

   cannot 
  be 
  directly 
  affirmed, 
  however 
  strong 
  such 
  an 
  inference 
  

   at 
  present. 
  For 
  other 
  swimming 
  reptiles 
  with 
  straight 
  humeri, 
  

   scarcely 
  furnish 
  an 
  explanatory 
  analogy. 
  Such 
  have 
  lithe 
  

   instead 
  of 
  rigid 
  bodies 
  and 
  are 
  provided 
  either 
  with 
  a 
  caudal 
  

   fin 
  or 
  a 
  powerful 
  swimming 
  tail. 
  There 
  are, 
  however, 
  salient 
  

   facts 
  bearing 
  on 
  this 
  point. 
  

  

  Those 
  turtles 
  which 
  inhabit 
  dry 
  and 
  especially 
  rocky 
  locali- 
  

   ties 
  present 
  the 
  most 
  specialized 
  land 
  humerus, 
  while 
  species 
  

   of 
  intermediate 
  habits 
  like 
  Chelopus 
  are 
  marked 
  by 
  the 
  gen- 
  

   eralized 
  type 
  of 
  land 
  humerus. 
  In 
  passing 
  however 
  from 
  such 
  

   to 
  the 
  more 
  and 
  more 
  aquatic 
  turtles 
  there 
  is 
  an 
  increasing 
  

   freedom 
  of 
  leg 
  movement. 
  Footings, 
  or 
  the 
  fulcra 
  of 
  loco- 
  

   motion, 
  as 
  used 
  by 
  the 
  animal 
  become 
  more 
  and 
  more 
  equally 
  

   distributed 
  about 
  the 
  body 
  and 
  undoubtedly 
  furnish 
  a 
  sufficient 
  

   explanation 
  for 
  humeral 
  straightening 
  in 
  the 
  transitional 
  forms, 
  

   and 
  the 
  fundamental 
  difference 
  in 
  curvature 
  between 
  the 
  

   humeri 
  of 
  the 
  existing 
  slow-moving 
  land 
  and 
  swift-moving 
  

   marine 
  turtles. 
  

  

  Once 
  having 
  entered 
  salt 
  water, 
  a 
  new 
  course 
  of 
  evolution 
  

   began. 
  The 
  early 
  transitional 
  Testudinates 
  so 
  far 
  as 
  the 
  fossil 
  

   record 
  indicates 
  were 
  mainly 
  of 
  inconspicuous 
  size. 
  They 
  

   were 
  yet 
  to 
  undergo 
  development 
  into 
  large 
  and 
  powerful 
  

   more 
  or 
  less 
  predatory 
  forms 
  invading 
  the 
  high 
  seas. 
  That 
  

   such 
  an 
  evolution 
  required 
  a 
  constantly 
  increasing 
  differentia- 
  

   tion 
  in 
  humeral 
  outline 
  and 
  musculature, 
  is 
  evident. 
  Small 
  

   creatures 
  of 
  varied 
  feeding 
  habits 
  living 
  in 
  quiet 
  waters, 
  taken 
  

   together 
  with 
  the 
  fierce 
  turtles 
  of 
  the 
  Cretaceous 
  seas 
  and 
  the 
  

   huge 
  predaceous 
  Dermochelys 
  sweeping 
  the 
  ocean 
  and 
  baffling 
  

   with 
  its 
  currents 
  and 
  storms, 
  suggest 
  the 
  utmost 
  variation 
  in 
  

   humeral 
  impacts, 
  stresses, 
  and 
  strains. 
  

  

  The 
  only 
  basis 
  for 
  variant 
  or 
  perfecting 
  leg 
  power 
  is 
  a 
  

   variant 
  or 
  perfecting 
  musculature, 
  and 
  the 
  only 
  support 
  of 
  this 
  

   correlated 
  skeletal 
  modification. 
  The 
  nature 
  as 
  well 
  as 
  the 
  

   extent 
  of 
  such 
  modification 
  in 
  the 
  Testudinata 
  will 
  be 
  still 
  

   better 
  understood 
  after 
  a 
  comparative 
  study 
  of 
  the 
  humeral 
  

   myology 
  of 
  the 
  order. 
  It 
  is 
  quite 
  obvious 
  at 
  least 
  that 
  the 
  

   humerus 
  furnishes 
  as 
  readily 
  distinguishable 
  and 
  as 
  important 
  

   characters 
  as 
  the 
  carpus. 
  

  

  