, RODENTIA—SACCOMYINAE—DIPODOMYS. 409 
and metacarpal are five in number, and distinct throughout, although the inner in each is quite 
small, and bears a diminutive, scarcely appreciable claw. The clavicles are perfect ; the tibia and 
fibula united below, at about the middle of the former. 
In Wiegmann’s Archiv for 1846,page 172, Dr. A. Wagner gives a detailed account of a new 
genus of rodents from Mexico, called by him Macrocolus. (Species, M. halticus.) Although 
Wagner expressly states that there were no external cheek pouches in his specimen, and that, 
in consequence, it could not belong to Gray’s genus, Dipodomys, yet the coincidence in every 
other respect, skull, teeth, skeleton, and external form, isso very intimate as to render it almost 
certain that the cheek pouches must have been overlooked, especially as we are particularly 
informed that the specimen was in very defective condition as preserved in alcohol. The species 
was probably identical with that described by Gray, viz: D. phillipii, which appears to be the 
one common in Mexico. The entire skeleton, with details of the skull, was afterwards figured 
by Wagner, in Math. Phys. Abhandlungen der K. B. Akademie of Munich, V, 1848, plate vii. 
The determination of the species of Dipodomys is a matter of much uncertainty, in our ignor- 
ance of the true value of the characters upon which the species have been based. These are 
the amount of development of the antitragus, the color of the fur, the comparative length 
of the tail, and the color of the terminal portion of the latter. The uncertainty has been 
increased by the determinations having been made on skins stuffed to greater or less than natural 
size. Now, judging from analogy, there may be a considerable variation in the proportions of 
body and tail without involving specific diversity, and the color of the body may vary with age 
and season from light to dark, as it does in other rodents, and particularly in the gophers, 
(Geomys and Thomomys.) The specimens before me present every connecting link between the 
two extremes of coloration of the tail, and I would not be at all surprised if our North American 
species were to be properly reducible to two, one a short-tailed species from New Mexico and the 
Rocky Mountains, and the other a long-tailed one from California and Oregon. At the same 
time it is not yet certain that either of these species, or which one, is the same with the Dipodomys 
phillipti of J. E. Gray.? 
The figure and part of the description of Audubon and Bachman are taken from this identical 
specimen described by Gray, which resembles the white tipped specimen from Fort Reading, 
although the colors on the plate are more yellowish than Gray’s description will warrant. 
Whatever the number of species, all hitherto detected in North America belong to the two 
following sections: 
Section I. Hind foot not exceeding 1.50 inches, usually appreciably less ; about’one-third the 
length of head and body. Tail vertebree about 1} times the length.of head and body in nature; 
rarely exceeding 5 inches, never 5}. JD. ordii. 
Section II. Hind foot, 1.60 inches, sometimes more; always considerably exceeding 1.50; almost 
half as long as the head and body in the fresh specimen. Tail vertebrae 1} times as long as the 
head and body, always exceeding 5} inches; usually from six to seven inches. D. phillipii, 
D, agilis. 
The species of Dipodomys are found from Real del Monte, Mexico, along the high lands as far 
? Dipodomys phillipit, Gray.—Gray brown, with longer black hairs; sides sandy; sides of the nose, spot near the base of 
the ears, band across the thigh, and beneath, pure white ; nose, spot at the base of the long black whiskers, and at the 
base of the tail, black ; tail, black brown, with the band on each of its sidesand tip white ; penis ending in a long spine. 
Length of head and body, 5 inches; tail, 6} inches; hind feet, 1} inch. Inhabits Real del Morte, Mexico. Specimen in 
British Museum. Annals and Magazine of Natural History, VII, August, 1841, 621. 
52 L 
