472 U. S. P. R. R. EXP. AND SURVEYS ZOOLOGY GENERAL REPORT. 



HESPEROMYS MYOIDES. 



Hamster Mouse. 



Cricettu myoidea, GAPPER, Zool. Jour. V, 1830, 204 ; tab. x. 

 ' f Mu lewxpus, THOMPSON, Nat. Hist. Vt. Appendix, 1853, 13. 

 IHesperomytgroeilu, LECONTK, Pr. A. N. Sc. Phila. VII, Dec. 1855, 442. 



SP. CH. Size of //. leucopus or larger. Tail vertebrae generally .25 of an inch longer than head and body, with a decided 

 pencil at the end. Jaws with moderate cheek pouches. Color above, cinnamon brown, lined with dusky ; ears and upper 

 part of tail, similar. Under parts and feet, pure white, the color extending over the whole fore leg. Immature speci 

 mens fuscous, much as in H. leucopus. 



My attention was first called to this species by noticing that all the white-footed mice from 

 near Burlington, Vermont, had much longer tails in proportion than those from Middleboro', 

 Massachusetts. It then occurred to me to see if the former were in any way related to the 

 Cricetus myoides of Gapper, and, much to my astonishment, I found decided indications of 

 cheek pouches in all the alcoholic specimens examined. I then investigated a considerable 

 number of Middleboro' specimens, and in none could -I detect the slightest indication of 

 anything of the kind. 



These pouches are not very deep in the alcoholic specimens, as might naturally be expected 

 from the highly contractile character of their walls. In fresh specimens, according to Gapper, 

 they extend to the ear ; here, however, they only reach back as far as the posterior edge of the 

 eye, or about .30 of an inch from the edge of the mouth. They open on each side of the fleshy 

 palate, between the incisors and the molars, and pass obliquely backwards. The walls are 

 thickened and corrugated, probably by immersion in alcohol ; naked, or with a scattered hair 

 here and there. In the bottom of one were a few bits of a blackish substance. 



In external appearance there is very little to distinguish this species from H. leucopus from 

 Middleboro'. The ears are very similar, except, perhaps, a little narrower and higher, 

 and less rounded above. The antitragus is long and quite well developed. The nose and feet 

 are precisely similar, except, perhaps, that the tubercles of H. myoides are larger. The tail 

 only is generally longer than the head and body instead of being shorter. It has also a long 

 pencil of hairs at the tip. 



I fear I shall find it quite difficult to define this species so that it will be recognized without 

 anatomical examination. The colors, however, differ in some points from those of H. leucopus 

 resembling more those of H. aureolus. I have never seen so vivid a yellowish brown in 

 H. leucopus as pervades the upper parts of H. myoides. There is more or less of a dusky bam! 

 along the back, not so conspicuous, perhaps, as in H. leucopus. In the only two adult or red 

 colored specimens before me the membrane of the ear and the upper part of the tail, instead of 

 being fuscous brown, are yellowish or cinnamon, a feature not seen in the other. In the mor 3 

 plumbeous or grayish young, however, this character is inappreciable. The entire under parts 

 of body and tail and the feet are snowy white, the line of demarcation passing high up on th3 

 sides, leaving the whole outside of the fore leg white. 



More immature specimens, instead of the yellowish brown, are more fuscous, as in the 

 H. leucopus, the dusky color extending even to the wrist. 



As far as I can discern there is nothing whatever in the skull and teeth of this species ia - 

 any way different from that of Hesperomys leucopus. It bears no resemblance to that of 

 Cricetus, and as the only common character lies in the mutual possession of cheek pouches, t ' 



