﻿Wieland 
  — 
  Evolution 
  of 
  the 
  Testudinate 
  Humerus. 
  419 
  

  

  As 
  compared 
  with 
  the 
  preceding 
  or 
  thalassoid 
  form 
  the 
  chief 
  

   variation 
  is 
  seen 
  in 
  the 
  distal 
  retreat 
  of 
  the 
  radial 
  process. 
  No 
  

   distinct 
  approximations 
  to 
  any 
  of 
  the 
  land 
  forms 
  remain. 
  The 
  

   entire 
  development 
  is 
  in 
  an 
  ecto-enteral 
  plane. 
  The 
  strong 
  

   dorso-ventral 
  sigmoid 
  curve, 
  and 
  sharp 
  angle 
  between 
  the 
  

   radial 
  and 
  ulnar 
  crests 
  as 
  seen 
  in 
  the 
  parachelic 
  and 
  chelic 
  

   forms 
  has 
  disappeared. 
  

  

  This 
  type 
  scarcely 
  shows 
  as 
  much 
  constancy 
  of 
  form 
  as 
  the 
  

   generalized 
  land 
  humerus, 
  but 
  is 
  yet 
  sufficiently 
  uniform 
  to 
  

   raise 
  any 
  fairly 
  well 
  marked 
  differences 
  to 
  generic 
  value. 
  

  

  Yariations 
  of 
  this 
  humerus 
  may 
  very 
  well 
  be 
  illustrated 
  by 
  

   mentioning 
  certain 
  fossil 
  forms. 
  Of 
  primary 
  interest 
  is 
  the 
  

  

  14 
  

  

  15 
  

  

  16 
  

  

  Fossil 
  Thalassic 
  humeri 
  of 
  nearly 
  related 
  genera 
  derived 
  more 
  directly 
  from 
  the 
  

   thalassoid, 
  form. 
  Cf 
  . 
  figs. 
  6, 
  9 
  and 
  1 
  0. 
  

  

  Figure 
  13. 
  — 
  Desmatochehjs 
  Lowii 
  WillistoD, 
  f 
  natural 
  size. 
  From 
  Fort 
  Benton 
  

  

  Cretaceous. 
  Outlined 
  from 
  Williston 
  (20). 
  

   Figures 
  14-16. 
  — 
  Atlantochelys 
  Mortonii 
  Agassiz, 
  x 
  about 
  ^. 
  From 
  the 
  Green 
  

  

  sand 
  of 
  Burlington 
  County, 
  New 
  Jersey. 
  Outlined 
  from 
  Leidy 
  (12). 
  

  

  historic 
  Atlantochelys 
  Mortonii 
  Agassiz 
  described 
  in 
  1849 
  (1). 
  

   Subsequently 
  Leidy 
  referred 
  this 
  fossil 
  humerus, 
  together 
  with 
  

   that 
  of 
  ISTeptunochelys 
  (Protostega) 
  tuherosa 
  mentioned 
  above, 
  

   to 
  the 
  Mosasauria 
  (12). 
  Later 
  Cope 
  referred 
  it 
  to 
  Protostega 
  

   (6). 
  Finally 
  it 
  was 
  validly 
  rehabilitated 
  by 
  Leidy 
  (13). 
  The 
  

   strongest 
  resemblance 
  of 
  this 
  important 
  form, 
  not 
  before 
  

   pointed 
  out 
  I 
  believe, 
  is 
  to 
  Desmatochehjs 
  Lowii 
  Williston 
  

   (20, 
  21), 
  the 
  main 
  skeletal 
  characters 
  of 
  which 
  are 
  fortunately 
  

   known. 
  The 
  resemblance 
  is 
  sufficiently 
  close 
  to 
  suggest 
  allied 
  

  

  