The author in the first instance described the characters of the teeth of Mesoplodon Layardii from two specimens which had been collected during the expedition of H.M.S. “Challenger,” under the scientific superintendence of Sir C. Wyville Thomson. The one specimen, a young animal, under 14 feet in length, was obtained at Port Sussex, East Falkland Islands, by Mr H. N. Moseley, F.R.S.; the other, an adult skull, was procured at the Cape of Good Hope.

The teeth of the younger animal, two in number, were imbedded in their alveoli in the lower jaw. Each tooth consisted of a small triangular denticle or crown projecting outwards, and slightly upwards from the middle of the upper border of the fang.