The Sewellel is described as having very much the same habits as the Prairie Dog, living in society, burrowing very readily in the ground, and feeding on roots and berries. Their companies, however, seem to be much smaller than those of the Prairie Dog, and they are said chiefly to frequent spring-heads in rich, moist places. They are described as having the curious habit of neatly cutting off some herb or plant, which, when packed in bundles, they lay out and expose to the sun to dry; this is probably for the purpose of storing for winter consumption. It seems to be uncertain whether the Sewellel is torpid during the winter, but probably in this respect it varies according to local conditions or the coldness of the seasons. Dr. Suchley believes that the Sewellel has several litters of young during the season. The Indians trap them, and esteem them very highly as food. Cloaks or blankets are made of their skins, which are sewn together with fibres derived from the sinews of the Elk and Deer. A robe described by Sir John Richardson was composed of twenty-seven skins. FAMILY IV.--CASTORIDÆ. Unlike as the Beaver may be to a Squirrel, it yet presents many characters which prove that its nearest affinity is to the animals which compose the group Sciuromorpha. This relationship has indeed been overlooked by many zoologists, but Mr. Alston and Mr. Allen have clearly shown that Professor Gervais was right in placing the Castoridæ in close juxtaposition with the Squirrels. The peculiarities which make the apparent discrepancy so striking are indeed chiefly those by which the Beaver is adapted to an aquatic life. [Illustration: MOLAR TEETH OF THE BEAVER.] The Beaver, which is the sole living representative of this family, is a more powerful animal than any of the preceding, and his incisor teeth and the means of working them are especially well developed. The head is large and the skull massive, and furnished with a distinct median (sagittal) crest for the insertion of the strong muscles which move the lower jaw. There are no post-orbital processes. There are four molars on each side in each jaw, and these are nearly similar in size and structure; but, contrary to what we have seen in the preceding groups, the first molar is the largest, and the others diminish in size towards the hinder end of the row. The series of teeth in the two sides of the mouth converge toward the front; and the teeth themselves, which are for a long time rootless, and only close up to form a simple root when the animal grows old, show three folds or loops of enamel on one side, and a single fold on the other: the three folds entering from the outer surface of the tooth in the upper jaw, and from its inner surface in the lower.