SOUND-TRANSMITTING APPARATUS OF CAUDATA 353 



plate is formed by such growth cannot be definitely stated, but 

 certain Umits for its contribution may be judged by two obser- 

 vations upon a young adult 85 mm. long in which the plate is 

 not yet completed. The first point to be noted is that the stain- 

 ing reaction of the stylar and fenestral cartilage is different and 

 apparently consistent in each. From this it appears that only 

 a small portion of the plate surrounding the end of the stylus is 

 formed by the spreading of the stylar tissue over the fenestral 

 membrane and this only about the very end of the stylus. The 

 second observation of importance is that concerned with the 

 connection of the fenestral plate and otic capsule. An isthmus 

 fenestralis is present, typical in location and relations. 



In urodeles there are only two different connections between 

 the fenestral plate and the margin of the fenestra. One of these 

 is typified by the Amblystomidae, the other by the Plethodont- 

 idae, both described above and determined by development. In 

 the amblystomids the connection is produced by growth from the 

 plate toward the ear capsule, in the latter by growth from the 

 ear capsule into the fenestral membrane. The two are distinct 

 as regards location and mode of development and are consistent 

 within the groups in which they obtain. The evidence, so far as 

 it may be considered as such, favors the conclusion that the isth- 

 mus in Amphiuma is of the plethodontid type. Further evi- 

 dence for this view is found in the incomplete state of the plate 

 in the young adult studied. The band of cartilage below the 

 stylus (fig. 28, V,a.) represents the growing ventral arm from the 

 isthmus, while that above is to be considered the dorsal arm as 

 in Spelerpes. Except for the few developmental observations 

 actually made and the conclusions deduced from comparisons 

 with other urodeles, the columellar nature of the fenestral plate 

 in Amphiuma would not be questioned. All that has been gained 

 from the present study forces the conclusion that the sound- 

 transmitting apparatus of this species resembles the plethodontid 

 type more than the amblystomid type, and in this respect Am- 

 phiuma must be viewed as intermediate between Necturus and 

 the Plethodontidae. 



