HIPPOPOTAMUS, L.—The discovery, in lacustrine and fluviatile deposits of Europe, of the remains of an amphibious genus of Mammal now restricted to African rivers, gives scope for speculating on the nature of the land which, uniting Eng land with the Continent, was excavated by lakes and inter sected by rivers, with a somewhat warmer temperature than at present, to judge by a few S. European shells which occur in the fresh-water formations—e. g., at Grays, Essex, where remains of the large extinct Hippopotamus major have been found. The specimen of the lower jaw (fig. 121) was dis covered in similar deposits on the Norfolk coasts. Other localities are specified in the writer's" History of British Fossil Mammals." The first premolar has a simple subcompressed conical crown, and a single root ; it rises early, and at some distance in advance of the second premolar, and is soon shed ; the other premolars form a continuous series with the true molars in the existing species, but in the Hippopotamus major the second premolar is in advance of the third by an interval equal to its own breadth. This and the fourth premolar re tain the simple conical form, but with increased size, and are impressed by one or two longitudinal grooves on the outer surface, which, when the crown is much worn, give a lobate character to the grinding surface.. The true molars are pri
marily divided into two lobes or cones by a wide transverse valley, and each lobe is subdivided by a narrow antero-pos tenor cleft into two half cones, with their flat sides next each other ; the convex side of each half cone is indented by two angular vertical notches, bounding a strong intermediate pro minence. When their summits begin to be abraded, each lobe, or pair of deco icones, presents a double trefoil of enamel on the grinding surface, as shown in fig. 120 ; when attrition has proceeded to the base of the half cones, then the grinding surfaces of each lobe presents a quadrilobate figure. The crown of the last molar tooth of the lower jaw is lengthened out by a fifth cone, developed behind the two normal pairs of half cones, and smaller in all its dimensions.
The hippopotamus is first met with in pliocene strata. The remains of H. major have hitherto been found only in Europe ; they are common along the Mediterranean shore, and do not occur north of the temperate zone. In Asia this Lower jaw of Hippopotamus major (fresh-water Pliocene, Cromer, N,.rfolk). form of Pachyderm was represented, perhaps at an earlier period, by the genus He.r,aprotodon—essentially a tamus, with six incisor teeth, instead of four, in each jaw.