RODENTIA SACCOMYINAE DIPODOMTS. 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, is so 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 

 Kocky 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 

 phillipii of J. E. Gray. 1 



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 vertebrae about 1 times the length of head and body in nature; 

 rarely exceeding 5 inches, never 5. D. 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 If 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 



1 Dipodomys phillipii, GHAT. 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 bind on each of its sides and 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 Mor te, Mexico. Specimen in 

 British Museum. Annals and Magazine of Natural History, VII, August, 1841, 521. 



52 L 



