The whales have rounded heads and a roughly sausage-shaped profile that tapers at the tail end. Females grow to about 4 meters in length. Males tend to grow about half a meter longer, excluding the length of their tusks. The animals can weigh as much as 1.5 tons.

Narwhals have only two fully developed teeth, and often just the left one is visible. In males and a few females, that tooth grows into the remarkable tusk. It often becomes more than half as long as the rest of the narwhal's body. The other tooth usually stays embedded within the bone.

The elongating tooth spirals to the left, from a whale's-eye view. In the rare cases of double-tusked narwhals, perhaps 1 out of 500 males, the right tooth doesn't grow in a mirror image. Disdaining the symmetry of other animals' teeth, the narwhal grows another left-handed spiral.

http://www.sciencenews.org/articles/20060325/bob9.asp