\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.