﻿420 
  Wieland 
  — 
  Evolution 
  of 
  the 
  Testudinate 
  Humerus. 
  

  

  genera. 
  Cf. 
  figs. 
  13 
  and 
  14-16. 
  A 
  complete 
  skeleton 
  of 
  

   Atlantochelys 
  should 
  be 
  diligently 
  sought 
  for. 
  

  

  The 
  significance 
  of 
  such 
  variations 
  as 
  may 
  be 
  present 
  in 
  

   thalassic 
  humeri 
  is 
  shown 
  by 
  the 
  well 
  known 
  Protostega 
  gig 
  as 
  

   Cope 
  from 
  the 
  Niobrara 
  Cretaceous 
  of 
  Kansas, 
  and 
  the 
  more 
  

   recently 
  discovered 
  Archelon 
  ischyros 
  (mihi, 
  17-19) 
  from 
  the 
  

   overlying 
  Fort 
  Pierre 
  formation 
  of 
  South 
  Dakota. 
  Both 
  these 
  

   humeri 
  are 
  illustrated 
  in 
  figs. 
  8 
  and 
  17-19 
  in 
  text. 
  

  

  While 
  the 
  head 
  and 
  ulnar 
  crest 
  are 
  quite 
  similar 
  in 
  both 
  

   forms, 
  in 
  A. 
  ischyros 
  there 
  is 
  a 
  minor 
  development 
  of 
  the 
  

   radial 
  crest, 
  with 
  a 
  more 
  ental 
  position 
  of 
  the 
  deep 
  ectepicon- 
  

   dylar 
  groove. 
  Or 
  conversely 
  it 
  may 
  be 
  said 
  that 
  there 
  is 
  a 
  

   major 
  development 
  of 
  the 
  ectepicondylar 
  region, 
  obscuring 
  the 
  

  

  A 
  prominent 
  example 
  of 
  generic 
  variation 
  in 
  thalassic 
  humeri 
  approaching 
  the 
  

   parathalassic 
  type. 
  Cf. 
  figures 
  20 
  and 
  21. 
  Lettering 
  as 
  in 
  figure 
  1. 
  

  

  Figures 
  11 
  and 
  18. 
  — 
  Archelon 
  ischyros 
  Wieland, 
  from 
  the 
  Fort 
  Pierre 
  Cretaceous. 
  

  

  Left 
  outer 
  and 
  inner 
  view 
  respectively, 
  x 
  about 
  T 
  ^-. 
  

   Figure 
  19. 
  — 
  Protostega 
  gigas 
  Cope 
  from 
  the 
  Niobrara 
  Cretaceous. 
  Left 
  inner 
  

  

  view 
  x 
  about 
  i 
  Outlined 
  from 
  Case 
  (5). 
  

  

  ala-like 
  projection 
  of 
  the 
  radial 
  process 
  as 
  seen 
  in 
  Protostega, 
  

   and 
  leaving 
  the 
  ectepicondylar 
  groove 
  in 
  a 
  more 
  mesial 
  posi- 
  

   tion. 
  Such 
  a 
  difference 
  is 
  fundamental 
  in 
  the 
  thalassan 
  forms, 
  

   and 
  the 
  probability 
  that 
  it 
  will 
  be 
  accompanied 
  by 
  marked 
  

   cranial 
  and 
  other 
  variations 
  is 
  very 
  great. 
  This 
  instance 
  is 
  all 
  

   the 
  more 
  important 
  because 
  of 
  the 
  direct 
  descent 
  of 
  A. 
  ischyra 
  

   from 
  P. 
  gigas 
  (19). 
  

  

  The 
  Parathalassic 
  Humerus. 
  — 
  The 
  final 
  humeral 
  type 
  to 
  be 
  

   mentioned 
  is 
  the 
  parathalassic 
  seen 
  only 
  in 
  a 
  single 
  living 
  

   example, 
  the 
  most 
  aberrant 
  of 
  sea 
  turtles, 
  Derrnochelys. 
  In 
  

   this 
  humerus 
  dorso-ventral 
  curvature 
  of 
  the 
  shaft 
  has 
  wholly 
  

  

  