The Collaborative International Dictionary of English v.0.48:
Trionyx \Tri*on"yx\, n. [NL., fr. Gr. ? (see Tri-) + ? a
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 ferox syn.
Aspidonectus ferox) becomes over a foot in length and
is very voracious. Similar species are found in Asia