In September last, I drew the attention of the readers of the Geological Magazine (Decade III. Vol. II. pp. 412–425) to a singular group of vegetable-feeding aquatic animals, the Sirenia, represented at the present day by two genera, Manatus and Halicore, and by at most six species, three or four of which are probably only varieties. A peculiarity of this group of animals is that whereas the two living genera are distributed in the subtropical regions East and West of the African continent, and to its rivers and opposite coasts, the ancestors of the Halicore and Manatee (the Halitherium, Felsinotherium, and some ten other fossil genera of Sirenians) probably intermingled and extended 30° further north than at present from the West Indies and Carolina through England, Belgium, France, Germany, Italy, and North Africa, whilst Rhytina, the largest of them all, only became extinct 100 years ago on the shores of Behring's Island, Kamtchatka. The evidence which the fossil remains of Sirenia afford of a more northerly geographical extension of subtropical mammalia in Tertiary times, is abundantly confirmed by other genera, to one of which only, the Hippopotamus, I will here refer.