Some bones of a gigantic Rodent, indicating an animal as large as a full-grown Black Bear, obtained from Quaternary and Alluvial deposits of various parts of the United States, have been described under the name of _Castoroides ohioensis_, Mr. J. W. Foster, its first describer, having an idea that it was a great Beaver. It has generally been known as the “Fossil Beaver” of North America, but several authors have entertained doubts of the correctness of this designation, and Mr. Allen has lately made it the type of a special family, CASTOROIDIDÆ, which he regards as most nearly related to the Chinchillidæ. In the general aspect of the skull it resembles the Beaver, but in several details of structure approaches the Viscacha; while the structure of the molar teeth, which consist of a series of plates of dentine, completely enclosed by enamel, and held together by a thin coating of cement, occurs elsewhere only in the Chinchillidæ, and in the last molar of the Capybara. Dormice as well as Squirrels disported themselves in the Tertiary woods and thickets of Europe, and remains of several species of MYOXIDÆ occur in various deposits in France, Switzerland, and elsewhere, from the Upper Eocene onwards. _Myoxus glis_, the Garden Dormouse, has been identified with some doubt from the caves of Lunel Viel; and this is also probably the species occurring in the Belgian bone-caves, and described as _Myoxus priscus_ by Dr. Schmerling. A species a little larger than the Dormouse occurs in Russian caves, and has received the name of _Myoxus fossilis_ from M. Fischer; and the most striking species of all is also a Post-Pliocene form, namely, the gigantic Dormouse of Malta (_M. melitensis_). This animal, which seems to have been about the size of a Guinea-pig, must have been excessively abundant in Malta, for its describer, Professor Leith Adams, says that “its remains are met with in abundance throughout the cavern and fissure deposits, up even to the superficial alluvium now in course of formation.” From older times we have evidence of the existence of a Dormouse, about the size of the common species, at the time of the deposition of the gypsum of Montmartre (Upper Eocene), in which a well-preserved skeleton of the animal has been found. The same deposit has furnished traces of a second rather larger species. The Miocene of Switzerland and of Sansan has also yielded species of _Myoxus_; and Professor Hermann von Meyer has recorded a Dormouse from the Miocene of Weisenau, under the name of _Brachymys ornatus_.