The skull resembles that of bears, but the grinding teeth are of Ungulate type, while the second incisors resemble those of rodents, and have persistent pulps. The femur has a third trochanter, and the feet resemble those of bears in being plantigrade and having pointed ungual phalanges, differing, however, in having the scaphoid and lunar distinct. _Order 5._ RODENTIA. The Rodents form a very large and well-defined group of mammals easily distinguishable by their peculiar dentition. Canines are absent, and the incisors are very large and curved, growing from persistent pulps. They are rectangular in section and are much more thickly coated with enamel on their anterior face than elsewhere; consequently, as they wear down they acquire and retain a chisel-shaped (scalpriform) edge. There is never more than one pair of incisors in the mandible, and except in the Hares and Rabbits, there is similarly only a single pair in the upper jaw. These animals are, too, the only rodents which have well developed deciduous incisors. There is always a long diastema separating the incisors from the grinding teeth. The grinding teeth, which are arranged in a continuous series, vary in number from two to six in the upper jaw, and from two to five in the lower jaw. The number of premolars is always below the normal, often they are altogether wanting, but generally they are 1/1. Sometimes the grinding teeth form roots, sometimes they grow persistently. The premaxillae are always large, and the orbits always communicate freely with the temporal fossae. The condyle of the mandible is elongated from before backwards, and owing to the absence of a postglenoid process to the squamosal, a backward and forward motion of the jaw can take place. The zygomatic arch is complete, but the jugal is short and only forms the middle of it. The palate is small, being sometimes, as in the hares, narrowed from before backwards, sometimes as in the mole-rats (Bathyerginae) narrowed transversely. The thoraco-lumbar vertebrae are usually nineteen in number. Clavicles are generally present, and the acromion of the scapula is commonly very long. The feet are as a rule plantigrade, and provided with five clawed digits. There are two main groups of Rodentia; the Duplicidentata, or Hares and Rabbits, which have two pairs of upper incisors, whose enamel extends round to the posterior surface; and the Simplicidentata, in which there is only a single pair of upper incisors, whose enamel is confined to the anterior surface. This group includes all the Rodents except the Hares and Rabbits. _Order 6._ CARNIVORA. The living Carnivora form a natural and well-marked group, but as is the case with so many other groups of animals, when their extinct allies are included, it becomes impossible to readily define them.