212 PROCEEDINGS OF THE NATIONAL MUSEUM vol. no 



forests, SO that in the Miocene they became discontinuous, with seg- 

 ments in North America and Eurasia (Cain, 1944, p. 107). Presum- 

 ably, the distribution of the ancestors of Euderma and Plecotus may 

 have become discontinuous at the same time, providing isolation for 

 major evolution within the group. 



The Eurasian segment developed into an evolutionary dead end, 

 Barbastella, which, though greatly modified in form, represents the 

 ancestral stock. Barbastella has a geographic range most remote from 

 the postulated Nearctic center of origin of the group. 



Genus Euderma 



Meanwhile, in the North American segment, evolution was rapid 

 and spectacular, directed toward auditory specialization. Euderma is 

 a derivative of the early stages of this evolution. It is the most 

 specialized, and, in many respects, the most primitive of the big-eared 

 forms. It must have had a long period of evolution along its own 

 line, distinct from that of the other members of the group. Whereas 

 Euderma has developed characters of its own, such as the connection 

 of the posterior basal lobe of the auricle with the tragus, unique colora- 

 tion, reduction of dentition, and specialization of the canines, among 

 the other big-eared forms (genus Plecotus) there have been only minor 

 variations of features common to all. The nature of the isolating 

 mechanism that allowed the divergence of the Plecotus and Euderma 

 stems is not apparent. Possibly Euderma was southern and wide- 

 spread during Pliocene and had its range restricted and displaced by 

 the more northern Plecotus during the Pleistocene glaciations. 



Genus Plecotus 



Subgenus Idionycteris 



The species of Plecotus seem to represent evolutionary stages. 

 P. phyllotis is most primitive and conservative, and most like Euderma; 

 P. auritus is somewhat more progressive; and P. mexicanus, P. rajines- 

 quii, and P. townsendii are most progressive. P. phyllotis presumably 

 diverged from the main stem while it still bore much resemblance to 

 the earlier derived Euderma. P. phyllotis failed to match the special- 

 izations of the later derived forms, and if abundance and area of 

 distribution be taken as measures of success, it definitely occupies a 

 position inferior to them. It seems to be extremely rare, and inhabits 

 a restricted area on the fringe of the ranges of P. mexicanus and 

 P. townsendii. On the other hand, the more progressive P. auritus 

 and P. townsendii are widely distributed, with ranges trans-continental 

 in scope, and are relatively abundant. P. auritus is even the most 

 abundant bat in some parts of its range. 



