﻿Wieland 
  — 
  Evolution 
  of 
  the 
  Testudinate 
  Humerus. 
  417 
  

  

  quite 
  distinct 
  early 
  marine, 
  or 
  thalassoid 
  humerus 
  there 
  is 
  no 
  

   very 
  well 
  denned 
  boundary. 
  However, 
  if 
  all 
  the 
  forms 
  were 
  

   known 
  there 
  would 
  be 
  no 
  hiatus 
  at 
  any 
  point, 
  especially 
  since 
  

   it 
  would 
  then 
  be 
  possible 
  to 
  determine 
  offshoots 
  from 
  the 
  main 
  

   line 
  of 
  descent. 
  

  

  The 
  Thalassoid 
  Humerus. 
  — 
  The 
  second 
  transitional 
  form 
  

   includes 
  humeri 
  of 
  turtles 
  which 
  had 
  probably 
  become 
  wholly 
  

   marine 
  in 
  habit, 
  but 
  retained 
  certain 
  features 
  of 
  their 
  semi- 
  

   marine 
  ancestors 
  with 
  chelicoid 
  humeri. 
  The 
  conversion 
  into 
  

   the 
  generalized 
  marine 
  form 
  is 
  however 
  completed 
  by 
  the 
  vari- 
  

   ations 
  seen 
  in 
  this 
  humerus. 
  The 
  drop 
  of 
  the 
  radial 
  crest 
  so 
  

   strongly 
  marked 
  in 
  succeeding 
  marine 
  turtles 
  begins, 
  while 
  the 
  

   angle 
  between 
  this 
  crest 
  and 
  the 
  ulnar 
  crest 
  tends 
  to 
  disappear, 
  

   the 
  latter 
  becoming 
  very 
  prominent 
  in 
  an 
  ecto-enteral 
  plane. 
  

   Cf. 
  figs. 
  6, 
  9, 
  and 
  10, 
  also 
  5-8 
  in 
  text. 
  

  

  10 
  

  

  li 
  

  

  12 
  

  

  Thalassoid 
  and 
  thalassic 
  turtle 
  humeri 
  (with 
  lizard 
  humerus 
  compared 
  with 
  

   figures 
  1-4). 
  Lettering 
  as 
  in 
  figure 
  1. 
  

  

  Figures 
  9 
  and 
  10. 
  — 
  Xeptunochelys 
  (Protostega) 
  htberosa, 
  x 
  i. 
  From 
  Cretaceous 
  

   near 
  Columbus, 
  Mississippi, 
  outlined 
  from 
  Leidy 
  (12). 
  See 
  note 
  p. 
  418. 
  

   Figure 
  11. 
  — 
  Tholossochelys 
  caretta. 
  Left 
  outer 
  view. 
  

   Figure 
  12. 
  — 
  Htloderma 
  suspectum 
  Cope. 
  Eight 
  dorsal 
  view. 
  

  

  The 
  humerus 
  of 
  I/ytoloma, 
  which 
  has 
  been 
  made 
  the 
  subject 
  

   of 
  a 
  suggestive 
  paper 
  by 
  Dollo 
  (8), 
  may 
  be 
  cited. 
  He 
  holds 
  

   this 
  form 
  to 
  be 
  essentially 
  " 
  chelydroid" 
  a 
  view 
  which 
  is 
  in 
  

   the 
  main 
  concurred 
  in 
  here. 
  For 
  convenience 
  in 
  the 
  present 
  

   classification, 
  the 
  term 
  thalassoid 
  may 
  as 
  well 
  be 
  used, 
  the 
  

   resemblance 
  to 
  the 
  humerus 
  of 
  the 
  Cheloniidm 
  being 
  fully 
  as 
  

   strong. 
  It 
  should 
  be 
  borne 
  in 
  mind 
  that 
  the 
  chelicoid 
  humeri 
  

   of 
  the 
  Acichelyidxe 
  intervene, 
  so 
  far 
  as 
  may 
  now 
  be 
  judged, 
  

  

  