SOFT TORTOISES. 



57 



dusirke in having mesoplastral bones (fig. 57) which extend com- 

 pletely across the lower shell to meet in the middle line. They differ 

 from living Pleurodira in that when the pubic bones of the pelvis 

 articulate with the xiphiplastral elements of the plastron, the union 

 is not by suture or anchylosis ; hence they are assigned to a distinct 

 group, the Amphichelydia. 



Sub-order IV. — Trionychoidea (Soft Tortoises). 



The Soft River Tortoises, or Mud-Turtles (family Triomjchidce, Case 10. 

 59), which retract the head and neck in a vertical plane with an 



Fig. 59. 



Young American Soft Tortoises (Trionyx ferox). (No. 226-) 



S-like flexure, after the manner of the Cryptodira, constitute a group 

 of equal rank with the latter. They are characterised by the flatness 



