SOUND-TRANSMITTING APPARATUS OF CAUDATA 373 



restrial existence. The absence of a free larval period in Pleth- 

 odon and others is to be looked upon as further progress toward 

 a strictly terrestrial existence. 



The relations and estates of the columella and operculum in 

 urodeles, when coupled with other studies, lead to the conclusion 

 that these animals, as a group, have not found favorable sur- 

 roundings in the terrestrial zone. One after another has re- 

 turned to the water permanently, and the relative duration of 

 this secondary aquatic period is reflected in the structure and 

 development of the animal. Others are now in the course of 

 their regressive radiation, while a few, such as Plethodon, Auto- 

 dax, and Hemidactylium, because of their small size and secre- 

 tive habits, have succeeded in the terrestrial struggle and exhibit 

 features of structure and life-cycle which show no regression, but 

 rather an advance in adaptation to the dry zone. 



STBIMARY 



1. In all caudate amphibia two elements, columella and oper- 

 culum, are present in the sound-transmitting apparatus. 



2. In the most generalized state these elements exist inde- 

 pendent of each other as in Amblystoma, Triton, Diemictylus, 

 Siren, and Salamandra. The columella, being useful in aquatic 

 life only, fuses in part with the ear capsule at transformation 

 (Amblystomidae, Salamandra) or completely (Triton, Diemicty- 

 lus, and probably Siren). The adult Cryptobranchidae, having 

 failed to complete the metamorphosis, has the sound-transmitting 

 apparatus in an arrested state of development, the columellar 

 element alone being present. 



3. In all of the other families (Necturidae, Amphiumidae, 

 Typhlomolgidae, Plethodontidae, Desmognathidae) the repre- 

 sentatives of the columella and operculum fuse to form a single 

 plate. In this fusion there is, throughout the series of families, 

 a gradual reduction of the columellar element from a state where 

 it forms a portion of the fenestral plate, as in Necturus, to one 

 where it becomes stylus only, as in Desmognathus. 



4. The morphology of this apparatus shows the affinities and 

 descent of the families as indicated in plate 6. 



JOURNAL OF MORPHOLOGY, VOL. 33, NO. 2 



