SOUND-TRANSMITTING APPARATUS OF CAUDATA 345 



The morphology of the sound-transmitting apparatus in 

 Gyrinophilus is obviously the same as in Spelerpes. 



Manculus quadridigitatus. In this species, so far as one can 

 judge from a study of the adult only, the plethodontid type of 

 sound-transmitting apparatus prevails. It appears that the 

 dorsal arm from the isthmus extends scarcely dorsad of the 

 stylus and that independent islands of cartilage do not form 

 during the developmental period, so that the membrane is 

 somewhat free in its dorsal extent. 



Hemidactylium is similar in all respects to Manculus. The 

 stylus, although firmly coossified with the fenestral plate, exhibits 

 a loose morphologic relation, recalling those in Gyrinophilus. 

 The two elements touch each other for a short distance and 

 bony tissue forms between them, but the identity of each is clear 

 at all levels. 



Batrachoseps. Among all the Plethodontidae examined Ba- 

 trachoseps is unique in its sound-transmitting apparatus. Of 

 the three characteristic features of this apparatus in the Ple- 

 thodontidae only one is evident in the adult. There is not the 

 slightest suggestion of a stylus or of a suspensorial connection 

 through a special cord of fascia. The isthmus fenestralis is 

 absent, leaving the plate freely suspended in the membrane. 

 There is, however, a well-developed M. opercularis. Further- 

 more, the plate is relatively short in its horizontal diameter. 

 Except for features of ossification in the capsule and plate itself, 

 the whole region might easily be identified as belonging to an 

 amblystomid form in which only the operculum is present. 

 Judging the structure and relations of this element from the adult 

 only, it appears to represent the definitive operculum of Ambly- 

 stoma. If this be true, its whole estate should be looked upon 

 as the parallel of what occurs in Triton and Diemictylus. The 

 available evidence points to such a conclusion. The relation 

 of this element to other cranial structures favors its designation 

 as operculum, although these relations might vary with such 

 changes as the elongation, flattening, or shortening of the head. 

 One relation which is here to be more relied upon is that with 

 parts of the internal ear. A survey of the position of the colum- 



