We have thus passed very briefly in review the fossil Rodents belonging to the two great sections of the order to which all its living species are to be referred; and it will be seen that while a knowledge of their existence is necessary to complete the history of the order, they present none of those peculiar characters which lend such interest to the fossil members of many other orders. There is, however, one fossil South American type to which we have yet to refer, as, by the curious combination of characters which it presents, it has long been somewhat of a puzzle to palæontologists, and although generally placed among the Rodents, its peculiarities are such that Mr. Alston found himself compelled to establish a third primary section of the order for its reception. According to M. Bravard, the first discoverer of this peculiar type, the Pliocene deposits of the Pampas of La Plata contain the remains of three species belonging to it; but the bones which have been sent to Europe, and which represent most parts of the skeleton, seem all to belong to a single species, which has been very fully described by M. Serres under the name of _Mesotherium cristatum_.[55] What distinguishes it at once from all other known Rodents is the presence _in the lower jaw_ of four incisor teeth, the second pair being very small and placed immediately behind the outer edge of the broad middle pair. The latter are peculiarly widened and compressed from front to back in both jaws, and their summits, instead of being worn to a sharp chisel-like edge as in ordinary Rodents, show an elongated ring of enamel surrounding a slightly depressed surface. Hence Mr. Alston denominated this section HEBETIDENTATA, or BLUNT-TOOTHED RODENTS. The enamel in all the incisors is continuous round the tooth. The molar teeth are rootless and curved, the convex side being directed outwards, contrary to what occurs in other Rodents. They are surrounded by enamel, and show re-entering folds which differ in the two jaws. Their number on each side is five in the upper and four in the lower jaw. The skull is massive, with enormously-developed sagittal and occipital crests, the latter of which run forward so far as to join the zygomatic arches; and these crests rise so high that the upper surface of the actual brain-case is entirely concealed by them when the skull is looked at from the side. The lower jaw in its characters presents some resemblance to the same part in the Leporidæ; but it has the condyle for its articulation with the skull transverse, and fitting into a cavity of corresponding direction, a character which occurs in no other Rodent. Of the remainder of the skeleton we need only state that the animal possessed perfect clavicles; that the shoulder-blade and humerus somewhat resemble those of the Beaver; that the fibula articulated with the heel-bone; and that both front and hind limbs possessed five toes, some of which, judging from the form of the