180 Bulletin American Museum of Natural History. [Vol. Ill, 



tively below in the two forms, where, however, the basal white 

 zone is more restricted, the dark subterminal portion is less 

 deeply colored, and the gray tips of the hairs are longer. In 

 bulleri the ears and wing-membranes are blackish ; in mexicanus 

 they are brown. The difference in the general coloration of the 

 two forms is thus very striking, with no tendency to intergrada- 

 tions, so far as the material in hand is concerned. 



A comparison of the skulls shows well-marked cranial differ- 

 ences between the two forms, the skull in M. mexicanus being 

 much the larger, with the rostral portion much broader, and quite 

 different in general contour. 



The type of De Saussure's M. mexicanus came from Yautepec, 

 in the tierra caliente south of the City of Mexico, and hence from 

 a region physiographically very different from that inhabited by 

 M. bulleri. His description appears to agree perfectly with the 

 Tehuantepec specimens above referred to M. mexicanus. 



M. mexicanus, M. bulleri, and M. water housii (M. calif or nicus 

 is not at hand for comparison), differ very markedly from each 

 other in coloration, but only slightly in size or other external 

 features. The ears appear rather larger in both M. waterhousii 

 and M. bulleri than in M. mexicanus. M. waterhousii seems also 

 to be slightly the largest of the three forms, and M. bulleri the 

 smallest, the length of the forearm varying as follows : M. bulleri, 

 42 to 48 mm., averaging 45 ; M. mexicanus, 48 to 53, averaging 

 51 ; M. waterhousii, 51 to 53, averaging 52. In coloration M. 

 bulleri is much the darkest and M. mexicanus the lightest and 

 most rufescent, especially below. M. waterhousii is intermediate, 

 being darker and less rufescent than M. mexicanus, but not nearly 

 so dark above as M. bulleri, though of about the same tint below. 

 M. waterhousii, however, differs from both the others in having 

 the posterior third of the dorsal surface lighter and of a decided 

 yellowish brown, in contrast with the sooty grayish brown of the 

 rest of the dorsal surface, while in the others the whole dorsal 

 region is concolor. 



The length of the free portion of the tail varies in all ; while 

 generally only the last caudal vertebra is wholly free, not unfre- 

 quently half or more of the antepenultimate is also free. This is 

 notably the case in both M. waterhousii and M. mexicanus. It 



