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Trionychidae (soft-shelled turtles; order Chelonia, suborder Cryptodira) A family of highly aquatic turtles that occur in fresh, brackish, and occasionally sea water. The bony shell is regressed, and covered with vascularized, respiratory skin. The head has a proboscis and fleshy lips. The limbs are oar-like. There are 25 species, occurring in N. America, Asia, and Africa.