﻿Wieland 
  — 
  Evolution 
  of 
  the 
  Testudinate 
  Humerus. 
  421 
  

  

  disappeared, 
  and 
  there 
  is 
  an 
  extreme 
  retreat 
  of 
  the 
  radial 
  

   process. 
  The 
  ectepicondylar 
  groove 
  of 
  the 
  thalassic 
  humerus 
  

   is 
  here 
  deepened 
  into 
  a 
  foramen, 
  it 
  being 
  wholly 
  probable 
  that 
  

   this 
  change 
  has 
  been 
  correlated 
  with 
  the 
  retreat 
  of 
  the 
  radial 
  

   crest 
  in 
  this 
  and 
  preceding 
  forms. 
  It 
  will 
  be 
  noticed 
  that 
  the 
  

   humeri 
  of 
  Protostega 
  and 
  Arohelon 
  are 
  not 
  placed 
  in 
  the 
  same 
  

   class 
  with 
  this 
  form. 
  There 
  are 
  only 
  vague 
  resemblances 
  with 
  

   great 
  differences. 
  The 
  peculiarities 
  of 
  the 
  parathalassic 
  form 
  

   may 
  be 
  accentuated 
  by 
  mentioning 
  a 
  most 
  interesting 
  Pliocene 
  

   example 
  seen 
  in 
  Psephophorus 
  sccddi, 
  a 
  turtle 
  which 
  like 
  Der- 
  

   mochelys 
  is 
  provided 
  with 
  an 
  external 
  dorsal 
  and 
  ventral 
  bony 
  

   armor 
  (14). 
  In 
  figs. 
  20 
  and 
  21 
  it 
  will 
  be 
  noted 
  that 
  anteriorly 
  

   the 
  radial 
  process 
  of 
  Psephophorus 
  is 
  very 
  prominent, 
  while 
  

  

  Views 
  of 
  left 
  humeri 
  showing 
  intermediate 
  or 
  thalassic 
  position 
  of 
  the 
  radial 
  

   crest 
  and 
  ectepicondylar 
  foramen 
  (groove) 
  in 
  figures 
  22 
  and 
  23, 
  and 
  the 
  extreme 
  

   or 
  parathalassic 
  position 
  in 
  figures 
  20 
  and 
  21. 
  Letters 
  as 
  in 
  figure 
  1. 
  

   Figure 
  20. 
  — 
  Psephophorus 
  scaldi. 
  From 
  Pliocene 
  of 
  Belgium. 
  Left 
  ventral 
  

  

  view 
  x 
  about 
  |. 
  Outlined 
  from 
  Dollo. 
  

   Figure 
  21. 
  — 
  Dermochelys 
  coriacea. 
  Recent. 
  Left 
  ventral 
  view 
  x 
  \. 
  

   Figures 
  22 
  and 
  23. 
  — 
  Thalassochelys 
  caretta. 
  Left 
  ventral 
  and 
  ectal 
  views. 
  

  

  its 
  ventral 
  extension 
  is 
  either 
  disappearing 
  or 
  lacks 
  the 
  strong 
  

   development 
  seen 
  in 
  Dermochelys, 
  only 
  two 
  isolated 
  areas 
  for 
  

   ligamental 
  attachment 
  being 
  present. 
  Chelonian 
  paleontology 
  

   awaits 
  complete 
  knowledge 
  of 
  no 
  more 
  interesting 
  forms 
  than 
  

   the 
  several 
  less 
  known 
  species 
  of 
  Psephop 
  horus. 
  

  

  Conclusion. 
  

  

  Whenever 
  it 
  becomes 
  possible 
  to 
  compare 
  an 
  entire 
  series 
  of 
  

   Testudinate 
  humeri 
  and 
  tabulate 
  the 
  results, 
  definite 
  affirma- 
  

   tion 
  or 
  negation 
  of 
  causes 
  of 
  development 
  and 
  means 
  of 
  adapta- 
  

   tion 
  may 
  be 
  expected. 
  At 
  present 
  only 
  general 
  conclusions 
  

   can 
  be 
  reached, 
  and 
  these 
  be 
  but 
  briefly 
  stated. 
  

  

  