358 H. D. REED 



is any evidence, the only element in Siren is the operculum, the 

 general state of affairs resembling those of Triton. The single 

 element in Cryptobranchus is the columella, formed by the 

 growth of the stylar portion over the fenestral membrane in true 

 amblystomid fashion. The failure of the operculum to appear 

 in this form is due probably to its aquatic life. 



That which has been taken as the plethodontid type of sound- 

 transmitting apparatus is exemplified by Spelerpes. The plate 

 filling the fenestra is single and possesses three extrinsic connec- 

 tions: a) one with the suspensorium through the stylus; h) one 

 with the shoulder-girdle through the M. opercularis, and, c) one 

 with the ear capsule through the isthmus fenestralis. The plate 

 itseK is formed by both the growth of the isthmus into the fen- 

 estral membrane and the independent formation of cartilage 

 within the substance of the membrane which meets and fuses 

 with that of the invading isthmus. Figures 23, 24, 25, and 26 

 illustrate the mode of formation. The stylus is the only repre- 

 sentative of what in the amblystomid type is columella. Varia- 

 tions from this type in its perfect state, as just described, are 

 found in Necturus, Typhlomolge, and Amphiuma, where the 

 stylus expands, forming a varying portion of the plate itseK. 

 In Batrachoseps, where the stylus (columella) has disappeared 

 entirely, the isthmus is likewise absent, leaving a plate which is 

 free from the ear capsule and which represents, apparently, one 

 derived from independent cartilage formed in the membrane of 

 the fenestra. 



The characteristic features are the distinctness of the two ele- 

 ments, columella and operculum, in the amblystomid type and 

 the fusion of the two in the plethodontid type as well as a differ- 

 ence in the exact mode of origin. 



THE PHYLOGENETIC SIGNIFICANCE OF THE SOUND- 

 TRANSMITTING APPARATUS 



The following discussion is presented with a twofold object: 

 first, to outline the history of the tailed amphibia as one reads it 

 in the morphology of the sound-transmitting apparatus, and, 

 second, to emphasize the zoological bearing of the subject as a 



