Source : Webster's Revised Unabridged Dictionary (1913)

Trionyx \Tri*on"yx\, n. [NL., fr. Gr. ? (see {Tri-}) + ? a
   claw.] (Zo["o]l.)
   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}.

   Note: The common American species ({Trionyx, or Aspidonectus,
         ferox}) becomes over a foot in length and is very
         voracious. Similar species are found in Asia and
         Africa.