A long prominent tusk, with which some narwals are furnished, is considered as a horn by the whale-fishers, and as such has given occasion for the name of _unicorn_ being applied to this animal. This tusk occurs on the left side of the head, and is sometimes found of the length of nine or ten feet; according to Egedé, fourteen or fifteen. It springs from the lower part of the upper-jaw, points forward and a little downward, being parallel in its direction to the roof of the mouth. It is spirally situated from right to left, is nearly straight, and tapers to around, blunt point, is of a yellowish white colour, and consists of a compact kind of ivory. It is usually hollow from the base to within a few inches of the point. In a five feet tusk the diameter at the base is two and a quarter inches, and about three-eighths within an inch of the end. This external tusk is peculiar to the male, and there is another imbedded in the skull, on the right side of the head, about nine inches long. Two or three instances have occurred of male narwals having been taken, which had two large external tusks. The use of the tusk is ambiguous. It cannot be essential for procuring their food, nor for defence. Dr. Barclay is of opinion that it is principally a sexual distinction; and it appears not improbable that it is used in piercing the ice for convenience of breathing, without the animal being obliged to retreat to open water. If this latter supposition be correct, it affords another illustration of the wisdom of the great Creator, who has adapted in so many instances the organization of every animal to the locality which it inhabits.