32 BULLETIN 144, UNITED STATES NATIONAL MUSEUM 



with this almost universal distribution is a notably generalized type 

 of structure. The family Vespertilionidse, it is true, stands high 

 among the bats in the development of those general characters which 

 distinguish the Chiroptera from nonvolant mammals; but within 

 the family the genus Myotis is conspicuous for the retention of the 

 most primitive dental formula — lacking only one upper incisor, one 

 upper premolar, and one lower premolar of the complete eutherian 

 armature — and for the absence of the specialized features of cranial 

 structure and individual tooth form which characterize many of the 

 bat genera with limited range. More than 100 forms of Myotis 

 have been described. Thus the genus is one of the largest as well as 

 the most widely distributed of the entire order. For the most part 

 the species do not differ conspicuously from each other. They are 

 seldom noticeable in color and usually small in size. 



It is probable that a further study of these bats will show the de- 

 sirability of arranging them in several genera and subgenera. 

 Thomas (1915) has already used Leuconoe as a genus. At about the 

 same time (1916), Bianchi, dealing with the Siberian members of the 

 genus, applied the name Capaccinius, in a generic sense, to a group 

 containing the species capaccinii, dasycneme, longipes, taiwanensis, 

 pequinius^ and ricketti (the last of which he separated subgenerically 

 as Richettia). In the genus Myotis proper he recognized, in addi- 

 tion to the typical group, the subgenera Dichromyotis^ Ptemopterus^ 

 Parainyotis^ Isotiis, and Seh/sius. We have shown (p. 9) that the 

 Myotis vyyotis group has rather important dental characters which 

 may prove to separate it definitely from the rest of the genus ; but, in 

 our opinion, no final results can be attained without a detailed study 

 of the species inhabiting all parts of the world. 



A satisfactory arrangement of the descriptions which follow would 

 require that the least specialized members of the genus be first dealt 

 with, then those showing successively wider departure from a general- 

 ized condition. Such an ideal method is impracticable, however, on 

 account of the varying degi'ee of development shown by the structural 

 characters in the different species. Thus in Myotis lucifugus^ though 

 the ears are not unusually long, the feet are relatively large, while in 

 M. ccMfomicus the ears are elongated and the feet small; again in 

 M. keenii { = subulatus of recent authors) the feet are nearly as 

 large as in M. lucifugus but the ears are conspicuously lengthened. 

 In Myotis thysanodes^ the species with the most aberrant condition of 

 the molar cusps, the general structure shows no very high specializa- 

 tion. In other words, the different parts of the body vary independ- 

 ently, so that there is seldom an obvious correlation between their 

 degrees of development from one species to another. We have con- 



