NORTHERN SNAPPING TURTLE Elseya dentata (Gray, 1863)
More than one species may be recognised here. This tortoise is found in most river systems north of the tropic of Capricorn.
There appears to be a strong geographical variation in appearance and size of specimens. The largest specimens attain over 45 cm in size, (females), but the average adult size is about 30 cm. Adults of this species often have their heads' become disproportionately large, (macrocephaly), and are capable of giving a serious bite.
In the wild this species feeds on crustaceans, fish and possibly various tropical fruits that fall into the rivers that they inhabit. This tortoise lays the largest eggs of any Australian tortoise species, measuring 5.5 cm in length, at the end of the northern wet season. These hard shelled eggs incubate and hatch in the northern dry season. Like other species of tortoise in the genus Elseya, this species usually has a yellow plastron(under shell), but there is a strong tendency for this to turn black in older specimens, (melanism).

The above was from the book Australian Reptiles and Frogs by Raymond Hoser and now available on a fantastic CD-Rom along with a vast amount of other information, papers and the like on reptiles, frogs and other wildlife.