SOUND-TRANSMITTING APPARATUS OF CAUDATA 



355 



the isthmus is within the membrane, while that of the small 

 plate rests against it. These are the relations which should ob- 

 tain, respectively, for elements of otic and extraotic origin. In 

 following sections caudad, this relation is found to persist. The 

 extension of the extraotic part of the plate in the dorsal direction 

 is only slightly above the base of the stylus. Figure 18 shows 

 this feature and also that the stylus appears to spread out 



Fig. 17 Transection through the fenestra vestibuli of an adult Typhlomolge 

 at the level of the stylus columellae. C.I., canalis lateralis; Col., stylus columel- 

 lae (columella); Ec, ear capsule; Fe.m., fenestral membrane; Op., fenestral plate 

 (operculum); Sq., squamosum. 



Fig. 18 Transection of the ear region of Typhlomolge, passing through the 

 fenestra vestibuli caudad of the level of the stylus columellae. C.I., canalis 

 lateralis; Ec, ear capsule; Fe.ni., fenestral membrane; /./., isthmus fenestralis, 

 continuous with the lips of the fenestra; Op., fenestral plate (operculum), the 

 free membrane between it and the dorsal margin of the isthmus representing the 

 space into which the columella failed to spread; Sq., squamosum. 



slightly as a funnel-shaped element. Taking together what evi- 

 dence there is, both direct and deduced, it appears that the fen- 

 estral plate is double ; the stylus and a small portion of the plate 

 being extraotic, and, therefore, columella, while the isthmus and 

 bulk of the plate represents tissue formed within the membrane 

 and, therefore, otic in nature. In principle it agrees with the 

 plethodontid type. 



