SOUND-TRANSMITTING APPARATUS OF CAUDATA 357 



though retaining its stylus. In those where total disappearance 

 obtains it has come about through loss of stylus and complete 

 incorporation with the ear capsule. Inferentially, this accounts 

 for the complete effacement of the columella in Siren. Leaving 

 inference, however, out of the question, and drawing upon the 

 known facts only, it becomes obvious that the fenestral plate in 

 Siren is identical with the functional operculum of the adult 

 Amblystoma and, being such in its morphology and the apparatus 

 as a whole lacking the columellar portion, it argues for the simi- 

 larity of the sound-transmitting apparatus of Siren to that of 

 Triton and Diemictylus. 



SUMMARY 



A general survey of the sound-transmitting apparatus in the 

 tailed Amphibia reveals the existence of two morphological types. 

 One has been mentioned as the amblystomid type since it is 

 found in its most generalized state in the Amblystomidae. The 

 other is typified by the Plethodontidae and may, therefore, be 

 termed the plethodontid type. In the perfect amblystomid type 

 tw^o distinct elements are present in the completed apparatus. 

 One of these is columella which is extraotic in origin. It is com- 

 posed of a stylus which joins the suspensorium at its cephalic 

 end and at its caudal end spreads out over the fenestral mem- 

 brane, forming a plate. This structure serves during larval life. 

 At transformation the columella fuses with the ear capsule, and 

 a new element, the operculum, is cut from the walls of the ear 

 capsule to function during adult life. It occupies a position 

 caudad of the columella and comes into connection with the ceph- 

 alic end of the M. opercularis. The ' variation from the perfect 

 or generalized type results from a loss of function of the colum- 

 ella when the terrestrial existence is assumed. The Amblystom- 

 idae and Salamandra possess this type in its most perfect state. 

 The identification of the type resides in the operculum, it being 

 the constant and consistent one of the two elements. In the di- 

 rection of Triton and Diemictylus the columella develops, but 

 soon completely disappears through fusion with the ear capsule, 

 leaving only the operculum as a distinct element. So far as there 



JOURNAL OF MORPHOLOGY, VOL. 33, NO. 2 



