_Skull._—The skull is much like that of the squirrel’s, with the marked exception of having rootless molars, and the absence of post-orbital processes; the occipital crest is well-developed, the muzzle large, and nearly round. The bony orbits are largely developed; the auditory bullæ are small, but open at once into wide auditive tubes; the first molar is unusually small, oval, and situated against the antero-internal angle of the second. All the molars are rootless: the lower grinders are much like the upper, but somewhat longer and narrower. The molars in both jaws are situated much farther back than is usual, the centre of the skull being about opposite to the meeting of the second and third. The lower jaw is very singularly shaped, the inner edges of the molars on opposite sides being parallel; the descending ramus is bent, so as to be exactly horizontal behind, the postero-inferior edge being a straight line, nearly perpendicular to the vertical plane of the skull’s axis. The conformation of the incisor-teeth is admirably adapted to the purposes they have to fulfil; no carpenter’s gouging chisels are more effective tools than are these exquisitely-constructed teeth. It is essential that they should always have a sharp-cutting edge, in order to nip through the tough vegetable fibre on which the animal subsists; at the same time, strength and durability are indispensable. The Aplodontia has no whetstone or razor-grinder, to sharpen his tools when they grow blunt; but an Allwise Providence has so fashioned these wondrous chisels in all rodents, that the more they are used the sharper they keep; the contrivance is simple as it is beautiful. The substance of the tooth itself is composed of tough ivory, but plated on the outer surface with enamel as hard as steel. The ivory, being the softer material, of course wears away faster than the enamel; hence the latter, plating the front of the tooth, is always left with a sharp-cutting edge. The position this genus should occupy, in a systematic arrangement of the rodents, has always been a stumbling-block and a matter of doubt, in great measure attributable to the fact that but a single species of the genus is known, and very few specimens have hitherto been obtained. A fine male specimen has recently been set up in the British Museum collection, that I caught near my camp on the prairie. In many particulars the Aplodontia very nearly resembles the Spermophiles, particularly the prairie-dog (_Cynomys Ludovicciana_), but differs, as in the true squirrels, in the rootless molars and absence of post-orbital processes. In this respect it is allied to the beaver. It is quite impossible to assign it a well-defined and settled position, until a greater number of specimens are procured, from which more minute and careful examination of the bony and internal anatomy can be made. At present, however, it would appear to connect the beavers with the squirrels, through the Spermophiles.