110 PROCEEDINGS OF THE NATIONAL MUSEUM vol. no 



no instance are these bats sufficiently well known to determine the 

 teleological significance of these traits. 



The differences distinguishing the subgenera Idionycteris, Plecotus, 

 and Corynorhinus mainly concern the external configuration of the 

 nostrU, muzzle, and ears (table 2). Dental, cranial, and skeletal 

 differences are numerous but slight (table 3). In the following dis- 

 cussion, Plecotus townsendii is given as a typical example of the 

 subgenus Corynorhinus. Unless otherwise noted, statements regard- 

 ing it apply equally to the other Recent species of the subgenus, 

 P. mexicanus and P . rafinesquii. The subgenera Plecotus and Idio- 

 nycteris are monotypic, represented respectively by the species P. 

 auritus and P. phyllotis. Inasmuch as P. phyllotis is known to me 

 only from three specimens, statements concerning it must be regarded 

 as highly tentative. 



The dental formula is the same in the three subgenera, and, as may 

 be seen from the following comparisons, the individual teeth must be 

 closely scrutinized in order to detect differences. 



The secondary cusp of I' is absent or variable in P. townsendii, 

 usually present in P. mexicanus, and always present in P. rafinesquii, 

 P. phyllotis, and P. auritus. The space between I^ and the canine is 

 frequently, but not always, greater, and P^ averages larger in P. auri- 

 tus than in the other species. P* is consistently wider than long in 

 P. phyllotis and P. totonsendii; longer than wide in P. auritus. Be- 

 cause the metacone of M' is somewhat reduced in P. auritus, the 

 third commissure of M^ is equal to or shorter than the second in most 

 individuals, whereas in P. phyllotis and P. toumsendii the third com- 

 missure is normally equal to or longer than the second. Pi is usually 

 more robust in P. auritus and P. phyllotis than in P. townsendii, 

 considerably exceeding P3 in size. P4 is double-rooted in P. auritus; 

 single-rooted in P. phyllotis and P. townsendii. In P. phyllotis the 

 internal cusps of the mandibular molars are almost as high as the 

 external cusps; in the other species of Plecotus the internal cusps are 

 lower. 



An analysis of these observations on dentition reveals four features, 

 which, because each is consistent in at least one of the subgenera, may 

 be regarded as having primary taxonomic significance. These fea- 

 tures are the simplification in I\ M^, and P4 respectively, and the 

 specialization in P*. P. phyllotis exhibits the generalized (unsimpli- 

 fied and unspecialized) condition in three features, is specialized in 

 one (P4) ; whereas P. auritus is specialized in two (P* and M^) ; and 

 all of the species of the subgenus Corynorhinus are specialized in one 

 (P4) and one of the species in another (P). Three other features — 

 crowding of the upper incisor series and diminution of the size of P^ 

 and Pi — are inconsistent, but may be of some significance. In these 



