What is "soft-shell tortoise"

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 ferox syn. Aspidonectus ferox) becomes over a foot in length and is very voracious. Similar species are found in Asia and Africa.