SOUND-TRANSMITTING APPARATUS OF CAUDATA 367 



of all its members. During the early transitional period, when 

 the head must, for the greater part of the time, have rested upon 

 the substratum, the columella became perfected and served as 

 the communicating element between inner ear and jaws. As 

 amphibians became more and more terrestrial the head and 

 anterior end of the body was elevated above the substratum, in 

 consequence of the perfection in the use of the arms in terrestrial 

 locomotion. This elevation of the head rendered the columella 

 functionless, and, a second time in their evolution, these animals 

 were left without direct communication between the outer world 

 and the inner ear. As a compensation for this loss a second 

 auditory element, the operculum, was cut out from the ear cap- 

 sule behind the columella; this appropriated a slip from the ad- 

 jacent musculature, thus coming into communication with the 

 shoulder-girdle and establishing a new coupling between the 

 inner ear and the substratum along which disturbances niight 

 travel. The columella, being no longer of use, gradually fused 

 with the ear capsule, leaving only the operculum freely suspended 

 in the fenestra as the functional organ. The Amblystomidae 

 either live in their original type of habitat or have departed 

 only slightly from the terrestrial habitat of their ancestors, so 

 that more of the primitive features of the sound-transmitting 

 apparatus persist than in any other group. The Cryptobranch- 

 idae must have separated from the amblystomid stem fairly early, 

 since sufficient time has elapsed for the complete suppression of 

 the operculum. If this view be correct, the sound-transmitting 

 apparatus of the Cryptobranchidae is secondarily simple and the 

 Amblystomidae are, so far as the auditory structures are con- 

 cerned, the most primitive of living Caudata. It might be argued 

 that the Cryptobranchidae returned to an aquatic environment 

 before the operculum had become a functional part of the sound- 

 transmitting organs, in which event they would necessarily be 

 looked upon as primitive urodeles. The nature of their partial 

 transformation, however, and numerous features of structure do 

 not encourage such a conclusion. 



After the separation of the modern Cryptobranchidae and Am- 

 blystomidae from the main stem of this legion, all the other uro- 



