The Collaborative International Dictionary of English v.0.48:

Trionyx \Tri*on"yx\, n. [NL., fr. Gr. ? (see Tri-) + ? a
   claw.] (Zool.)
   A genus of fresh-water or river turtles which have the shell
   imperfectly developed and covered with a soft leathery skin.
   They are noted for their agility and rapacity. Called also
   soft tortoise, soft-shell tortoise, and mud turtle.
   [1913 Webster]

   Note: The common American species (Trionyx ferox syn.
         Aspidonectus ferox) becomes over a foot in length and
         is very voracious. Similar species are found in Asia
         and Africa.
         [1913 Webster]