The late professor B. S. Barton, in his Medical and Physical Journal, says, that a species of _Mus_ allied to the _M. bursarius_ of Shaw, is common in Georgia and Florida; that he examined a living specimen of this animal, and was convinced that it is no other than the Tucan of Hernandez, and the Tuza or Tozan of Clavigero. He says nothing of its size; but on the same page he remarks, "that another species of Mus, much larger [than] the Tuza, inhabits west of the Mississippi about latitude 30°, of which very little is known." Dr. Barton was aware that the cheek pouches, in the figures given by Shaw, are represented in an inverted position, but not having seen specimens from the trans-Mississippi country, he was unacquainted with their specific identity with those of Canada, from which those figures were drawn. In our zoological reports to Major Long, in the year 1819, the specimens which we found on the Missouri were recorded under the name of _bursarius_ of Shaw. Coxe, in his description of "Carolana called Florida, and of the Meschacebe," 1741, mentions a "rat with a bag under its throat, wherein it conveys its young when forced to fly." Several other writers have noticed these animals, of whom Dr. Mitchell, in Silliman's Journal, 1821, mentions the identity of specimens obtained beyond Lake Superior, with the M. _bursarius_ of Shaw. The animals belonging to this genus are distinguished by their voluminous cheek-pouches, which are perfectly exterior to the mouth, from which they are separated by the common integument, they are profoundly concave, opening downwards, and towards the mouth. The _incisores_ which are not covered by the lips, but are always exposed to view, are strong and truncated in their entire width at tip; the superior ones are each marked by a deep, longitudinal groove near the middle, and by a smaller one at the inner margin. The _molares_, to the number of eight in each jaw, penetrate to the base of their respective alveoles, without any division into roots, as in the genus _Arvicola_, _Lepus_, &c., their crown is simply discoidal, transversely oblong oval, margined by the enamel, and in general form they resemble the teeth of a _Lepus_, but without the appearance either of a groove at their ends, or of a dividing crest of enamel; the posterior tooth is rather more rounded than the others, and that of the upper-jaw has a small prominent angle on its posterior face; the anterior tooth is double, in consequence of a profound duplicature in its side, so that its crown presents two oval disks, of which the anterior one is smaller, and in the lower jaw somewhat angulated. All the molares of the lower jaw incline obliquely forward, and those of the superior jaw obliquely backward.