Royal Society, Feb. 6.-“Memoir on the Osteology of Hyo-potamidae.” by Dr. W. Kowalevsky. The paper is intended to fill a deficiency in our knowledge of the extinct creation by giving a complete osteology of one family of the Paridigitate Ungulate. It has been supposed that fussil representatives of this family would exhibit a less reduced skeleton and a more complete number of digits than recent genera; yet such is not the case. The genera Anoploiherium and Liphodon present in their feet the same degree of reduction as in recent Ruminants, save the confluence in a canon-bone. Notwithstanding this, they have been considered the progenitors of the Ruminantia, from a deficiency in other forms. The present paper introduces a new form, known only by the teeth till now; these, the ffyopotamicics, vary considerably in specific and generic form, ranging from the Lower Eocene up to the Lower Miocene period, and in size from a rabbit to a hippopotamus. The Eocene species, except one termed diplopus, have not lost the lateral digit*, and are included in the genus Hyopotamus. The division of the Ungulata into Paridigitata and Imparidigitata must have occurred about the cretaceous period, as shown by the diversity exhibited by both groups from the lowest Eocene. The former, the Paridigitata, split very anciently, perhaps in the chalk, into those with tubercular, and others with crescentic teeth. These groups, once separa'e, kept entirely apart, but frequently followed parallel lines of descent. Following these two divergent lines of descent, both groups culminate at the present time in such forms as Pha-cochcerus and Dicotyles for one group, and the Bovida for the other, links between these being absent. The Paridigitata with crescentic teeth will be termed Par. selenodonta, and those with tubercular Par. ounodonta. To the first group belong Anoplo-theriuni, Liphodon, fiyopotavuis, and others, together wilh the existing ruminants, whilst the second embraces the Suina, Hippo-potamina and Entelodon. There is in some cases difficulty in deciding whether the teeth are tubercular or crescentic, the lobes being so thick.