176 Proceedings of the Biological Society of Washington. 



measured in the flesh by the collector, respectively: Total length, 205, 

 217 (261);* head and body, 100, 105 (120); tail, 105, 112 (141); hind foot 

 (estimated), 15, 16 (19.5). Skull of type: Greatest length, 30.4 (32.7); 

 zygomatic breadth, 17. 3 (18.9); anterior interorbital constriction, 4.3 

 (5); posterior interorbital constriction, 5.1 (6.1); width across postorbital 

 processes, 6.9 (7.6); breadth of braincase, 11.4 (13); nasals, 12.3 x 2.8 

 ( 13.8 x 3.4); palate length from gnathion, 16.1 ( 17.6) ; front of canine to 

 I nick of fourth upper molar, 1 1 . 75 ( 1 2.05) ; combined length of first, second, 

 and third upper molars, 5.6 (5.8). 



Remarks. — The disparity in size between this species and its nearest 

 relative Marmosa canescens is such that, while intergradation is probable, 

 it can not safely be assumed. The general grayish coloration is amply 

 sufficient fo distinguish gaumeri from mayensis, but there are also marked 

 cranial differences, gaumeri having strongly developed postorbital processes 

 set off by decided constrictions in front and behind, whereas mayensis 

 has only slight suggestion of these characters. The nearly parallel-sided 

 nasals of gaumeri also are distinctive. 



Marmosa mayensis sp. nov. 



Type from Izamal, Yucatan, Mexico. No. 19,094 Field Museum of 

 Natural History. Collected March, 1913, by G. F. Gaumer. 



Characters. — Similar to Marmosa mexicana, but paler in color, especially 

 the under parts which are pale creamy white rather sharply defined from the 

 color of .the upper parts instead of deep ochraceous practically confluent 

 with the color of the upper parts as in mexicana. General color of upper 

 parts cinnamon with a fine almost imperceptible mixture of dusky; sides 

 slightly paler than back and nearly clear cinnamon; eye ring intense 

 black, extending posteriorly to anterior base of ear; under parts creamy 

 white, the hairs self-colored except on the sides of the belly where they 

 have slaty bases; feet white; tail drabbish brown above, scarcely paler 

 below. 



Skull practically as in mexicana; palate rather short and rostrum thick- 

 ened; nasals moderately expanded posteriorly. 



Measurements. — Total length, 228; head and body, 110; tail, 118; ear, 15. 



Remarks. — Marmosa mayensis is the Yucatan representative of M. 

 mexicana just as M. gaumeri is that of M. canescens. The pale creamy 

 color of the under parts is widely different from the rich ochraceous burl' 

 of mexicana. M. canescens inhabits the rather arid region of the Isthmus 

 of Tehuantepec while M. mexicana seems to lie confined to more humid 

 regii ins in Oaxaca and YeraOuz. These two species have not been recorded 

 from the same locality. Therefore, the occurrence of both mayensis and 

 gaumeri at Izamal, well within the arid part of Yucatan, is of interest. 

 Whether their local habits are identical or not might be investigated witli 

 profit. 



* For convenience of comparison, the measurements of a topotype of M. canescens, 

 kindly lent by the U. S. Biological Survey, are placed in parentheses after those of 

 gaumeri. 



