The CASTORIDÆ, including at present only a single species common to the northern parts of both hemispheres, are represented by several peculiar fossil forms. Remains of the Common Beaver (_Castor fiber_) are not uncommon in peat bogs and other late superficial deposits both in Europe and America; and, according to Sir R. Owen, in association with those of the Rhinoceros, Mammoth, and Mastodon, even in the Fluvio-marine Crag (Newer Pliocene) of Norfolk. In Belgium its bones have been found in caves. Among the Mammals from the Upper Tertiaries of the Sivalik Hills, Messrs. Falconer and Cautley record a Beaver distinct from the existing species, although nearly allied to it. The skull of a great Beaver, one-fifth larger than that of the living species, was obtained many years ago by M. Fischer from sandy deposits on the shores of the Sea of Azov, and, as it differed in some peculiarities of the teeth from _Castor fiber_, was described by him as constituting a distinct genus under the name of _Trogontherium Cuvieri_. It is now regarded as a true Beaver, and named _Castor Trogontherium_. The British species, described and figured by Sir R. Owen from the Norfolk Forest bed under the name of _Trogontherium Cuvieri_, is, however, quite distinct, and belongs to the genus _Diobroticus_, characterised by having the third upper and first lower molar teeth with four enamel folds, and the rest only with two, most of the folds soon becoming isolated as the teeth wear down. This animal must have been nearly twice the size of the European Beaver. At a still earlier period--namely, in the Miocene--the family Castoridæ was represented, both in Europe and America, by some small species, nearly agreeing with the Beavers in dentition, but differing in the characters of certain bones of the skull. These form the genus _Steneofiber_. The largest (_S. viciacensis_), from the Miocene of the Allier, was about half the size of the Beaver; another (_S. sansaniensis_), from the fresh-water limestone of Sansan, was about as large as a Rat; an American species (_S. nebrascensis_), from the “Bad Lands” of Dakota, was rather smaller than a Marmot, and presented some resemblance to the Agoutis in the characters of the teeth; and a fourth species (_S. pansus_) occurs in the Santa Fé marls. _Eucastor tortus_, a species rather smaller than a Marmot, is described by Dr. Leidy as very nearly related to the Beaver. Its remains were found in loose sands of the Niobrara River, Wyoming. _Chalicomys_ and _Palæomys_ are genera doubtfully placed here. Their species occur in the Miocene and Pliocene of Europe.