SOUND-TRANSMITTING APPARATUS OF CAUDATA 361 



elements to form the definitive sound-transmitting apparatus. 

 The forms included within a given legion possessing a sound- 

 transmitting apparatus involving the same architectural prin- 

 ciples are considered as more closely related to each other than 

 to those of another legion. 



Since the M. opercularis occurs in more than half of the uro- 

 dele families, regardless of legion, and bears a direct relation to 

 the mode of life of the animal, it is to be considered as of physio- 

 logical rather than morphological import. This being its status, 

 it is obviously of no significance in a strictly phyletic consider- 

 ation. The whole question of descent and relationships of the 

 urodeles, so far as concerns the sound-transmitting apparatus, 

 hinges upon the elements composing the fenestral plate or plates 

 and their mode of origin. Thus, in the Amblystomidae, Sala- 

 mandra, Triton, and Diemictylus one finds the constant and in- 

 dependent operculum of the amblystomid type. They all agree 

 in this particular among themselves and differ from all other 

 groups. They may, therefore, be looked upon as of kin or having 

 descended from a common ancestral stem in which this morpho- 

 logical feature was well established and survived the vicissitudes 

 of change in such a way as to preserve its caste. Similarly con- 

 stant in this legion is the tendency of the columella to spread over 

 the outer surface of the fenestral membrane and form a plate 

 which is continuous with the stylus columellae. The variations 

 in the state of the columella in this legion appear to be correlated 

 with the extent to which the various forms have become terres- 

 trial. The tendency of the columella to fuse with the ear capsule 

 expresses the trend of modifications with regard to this element 

 in this legion. It is very apparent that the more terrestrial a 

 species becomes, or the longer the period in its descent during 

 which it has occupied the terrestrial zone, the more completely 

 is the columella fused with the ear capsule. The extreme in 

 this direction is found in Triton and Diemictylus, where complete 

 effacement of the columella has taken place. The tritons, it 

 will be noted, are terrestrial except at the breeding season. The 

 aquatic abode of Diemictylus viridescens in the mature state has 

 been secondarily acquired in more recent times, following a ter- 



