370 H. D. EEED 



plate in the Necturidae. As stated above, it points to a former 

 terrestrial existence for these forms, in consequence of which the 

 columella became reduced and fused with the rapidly forming 

 operculum, rather than with the ear capsule. The loss of the 

 M. opercularis is to be associated with the recently acquired aqua- 

 tic habit. There results, in the Necturidae, a single fenestral 

 plate which is perfectly free from the ear capsule and which bears 

 the outward form of that structure in the Cryptobranchidae, 

 but morphologically is quite different. The two families, judged 

 by these organs, are not closely related. The Necturidae are 

 considered as the most primitive of the plethodontid legion since 

 the columella spreads out over the fenestral membrane to a 

 greater extent than in any of the others of the group. This con- 

 dition is interpreted as reminiscent of the earliest type of sound- 

 transmitting apparatus. The family is, therefore, placed in plate 

 6 as an offshoot of the plethodontid stem, intermediate between 

 others of that group and the amblystomid legion. Such a re- 

 lationship is in harmony with the presence of lungs in the Nectur- 

 idae which they should not possess were they closer of kin to the 

 Plethodontidae. Especially significant in this connection are 

 Norris' ('11) observations upon the cranial nerves, which are 

 said, in their entirety, to reach a rather high degree of speciali- 

 zation, and, in their arrangement, to resemble both the pletho- 

 dontid and amblystomid types and especially the former. 



The other f amihes of the plethodontid group represent branches 

 which originate from a main stem leading back toward the place 

 of departure, of the Necturidae. In these families there is a 

 gradual reduction of the columella from a state where it spreads 

 slightly over the fenestral membrane, to one where, as in the 

 Plethodontidae and Desmognathidae, it is represented by stylus 

 only and takes no part whatever in the formation of the plate, 

 to the surface of which it becomes joined. The extent to which 

 the columella extends over the fenestral membrane is taken as 

 the index of specialization. Applying this criterion, the Amphi- 

 umidae stand nearest the Necturidae, agreeing with them and 

 differing from the others in the possession of lungs. 



