The narwhal and the white whale, or Beluga, which furnishes "porpoise-hide" for boots and laces, are both caught in northern seas, and form a closely allied pair, similar to one another in shape and colour (the one white, the other grey), and of moderate size, about 12 ft. long. They both feed on cuttle-fish and minute shrimps, but the Beluga has many teeth and the narwhal (with the exception of some rudimentary ones) only a single pair, and these in the front of the upper jaw. In the female narwhal their pair of teeth remain permanently concealed in the jaw bone, and so does the right side one of the male. But the left side tooth of the male grows to an enormous size, projecting horizontally in front of the narwhal to a length of seven or eight feet. It is a powerful weapon, and is formed of ivory spirally grooved on the surface. The narwhal was called "the unicorn fish" or "Monoceras" in ancient times, and its spirally marked tooth was confused with the horn of the terrestrial unicorn--the rhinoceros. Very rarely the right tooth of the male narwhal grows to full size side by side with the left tooth. A specimen showing this double-toothed condition is in the Natural History Museum. A most curious fact, quite unexplained as yet, is that the spiral grooving on both the teeth turns in the same direction; in both it is like a spiral staircase in mounting which (starting from the base implanted in the jaw) you continually turn to the right. Now, in all other animal structures which have a spiral growth and are paired--one belonging to the right side of the animal, the other to the left, as, for instance, the spirally marked horns of antelopes and the more loosely coiled horns of sheep and cattle--one of the pair forms a right-handed and the other a left-handed spiral. They are "complementary"; one is the reflection, as in a mirror, of the other. Why the narwhal's tooth does not conform to this rule is a mystery.