﻿416 
  Wieland 
  — 
  Evolution 
  of 
  the 
  Testudinate 
  Humerus. 
  

  

  Though 
  the 
  uniformity 
  with 
  which 
  this 
  humerus 
  ranges 
  

   through 
  many 
  genera 
  of 
  the 
  Oryptodira, 
  Pleurodira, 
  and 
  

   Trionychia, 
  render 
  it 
  as 
  yet 
  of 
  little 
  value 
  to 
  the 
  paleontol- 
  

   ogist, 
  excepting 
  the 
  Chelydran 
  variation 
  here 
  included, 
  

   although 
  somewhat 
  aberrant 
  and 
  suggestive 
  of 
  natatorial 
  types. 
  

   Examples 
  of 
  chelio 
  humeri 
  are 
  seen 
  in 
  the 
  genera 
  JEmys 
  ) 
  

   Chelopus, 
  Trionyx, 
  Podocnemis, 
  Chelydra, 
  Chelys, 
  and 
  others. 
  

  

  Dorsal 
  views 
  of 
  right 
  humeri 
  showing 
  descent 
  of 
  radial 
  process 
  as 
  noted 
  by 
  

   Dollo 
  (8). 
  Lettering 
  as 
  in 
  figure 
  1. 
  

  

  Figure 
  5. 
  — 
  Ghelydra 
  serpentina. 
  

  

  Figure 
  6. 
  — 
  Lytoloma 
  crassicostatum 
  Owen 
  (Lower 
  Landenien 
  (Lower 
  Eocene) 
  of 
  

  

  Erquellines). 
  

   Figure 
  7. 
  — 
  Ghelone 
  mydas 
  L. 
  

  

  Figure 
  8.—Protostega 
  gigas 
  Cope, 
  x 
  about 
  ^. 
  (Niobrara 
  Cretaceous.) 
  

   Figures 
  5-7 
  are 
  outlined 
  from 
  Dollo 
  (8), 
  figure 
  8 
  from 
  Cope 
  (6). 
  — 
  Figure 
  5 
  is 
  

  

  regarded 
  as 
  Chelic, 
  6 
  Thalassoid, 
  and 
  7 
  and 
  8 
  as 
  Thdlassic. 
  

  

  The 
  transitional 
  Water-types 
  of 
  the 
  Mesozoic. 
  

  

  The 
  Chelicoid 
  Form. 
  — 
  General 
  osteological 
  characters 
  in 
  

   addition 
  to 
  position 
  in 
  time 
  have 
  led 
  to 
  the 
  belief 
  that 
  the 
  

   straighter 
  shafts 
  noticed 
  in 
  certain 
  Jurassic 
  turtles 
  such 
  as 
  

   Acichelys, 
  are 
  those 
  of 
  primitive 
  oceanic 
  Chelonia. 
  As 
  these 
  

   humeri 
  yet 
  bear 
  a 
  distinct 
  resemblance 
  to 
  the 
  land 
  forms 
  they 
  

   may 
  be 
  described 
  as 
  chelicoid. 
  They 
  are 
  characterized 
  by 
  an 
  

   imperfectly 
  developed 
  head, 
  a 
  straightening 
  of 
  the 
  shaft 
  and 
  

   small 
  ulnar 
  and 
  radial 
  processes, 
  the 
  latter 
  being 
  placed 
  on 
  the 
  

   level 
  of 
  the 
  head. 
  Examples 
  are 
  seen 
  in 
  the 
  Acichelyidce 
  (14) 
  

   of 
  the 
  Wealden 
  and 
  Kimmeridge 
  ; 
  also 
  in 
  the 
  more 
  chelic 
  

   humerus 
  of 
  Acichelys 
  (Eurysternum) 
  Wagleri 
  (22), 
  which 
  has 
  

   the 
  lateral 
  proximal 
  crests 
  small, 
  but 
  the 
  articular 
  head 
  rela- 
  

   tively 
  large 
  with 
  some 
  straightening 
  of 
  the 
  shaft. 
  

  

  It 
  is 
  to 
  be 
  noted 
  that 
  these 
  intermediate 
  forms 
  are 
  the 
  most 
  

   difficult 
  to 
  place, 
  and 
  that 
  between 
  them 
  and 
  the 
  following 
  

  

  