These animals are of various sizes, some living on the land, and some
on the sea. They are remarkable for longevity. Mr. Murray says, "The
size to which this creature occasionally attains is quite monstrous. I
remember, some years ago, to have seen one, then semi-torpid, exhibited
near Exeter 'Change, London, which weighed several hundred weight. Its
shell was proportionably thick, and its other dimensions bore a
corresponding ratio. It was stated to be about _eight hundred_ years
old."