344 H. D. REED 



Although the general relations of the stylus are the usual ones 

 among urodeles, the course of the facial nerve deserves notice, 

 since it varies in certain details from those in other species, and 

 this variation is significant when compared with forms to be 

 discussed later. This nerve issues below the stylus, which is 

 normal. Figure 19 is a diagrammatic representation of the 

 course of the ramus jugularis VII in Amblystoma. It leaves 

 the main trunk just underneath the stylus and maintains a 

 horizontal course across the lower third of the columella and 

 operculum. In general, in its relation to fenestral structures, it 

 may be said to occupy a ventral position. The course of the 

 jugular branch in Gyrinophilus is illustrated in figure 22. Here 

 the R. jugularis VII leaves the main trunk in a dorsal direction 

 and keeps to a course along the dorsal edge of the fenestral 

 elements. 



A complete description of the development of the sound- 

 transmitting apparatus in Gyrinophilus would be a needless 

 repetition after what has already been written of Spelerpes. 

 A larva 82 mm. long exhibits all of the essential features for an 

 understanding of the morphology of the parts under consideration. 

 There are shown by a comparison of figure 25 of Spelerpes and 

 figure 31 of Gyrinophilus. The latter shows the isthmus fenes- 

 tralis as present and in its normal position. The estate of the 

 plate as a whole at this stage leaves no doubt as to the method 

 of cartilaginous invasion of the fenestral membrane from the 

 isthmus. A dorsal arm (D.a.) extends toward the stylus and a 

 ventral arm (V.a.) extends caudodorsad. The indented mar- 

 gins and fenestrae within the plate point to its formation by 

 growth and extension of the original invading arms and their 

 fusion with isolated areas of independently formed cartilage 

 within the fenestral membrane itself. The loose relation of the 

 stylus and plate in the definitive state is here accounted for; 

 only the extreme caudal end of the stylus comes into contact 

 with the plate early enough to admit of fusion, both because of 

 its morphological distinctness and an unchondrified area in the 

 fenestral membrane underneath it which persists into late larval 

 life. The long larval period here, as in Spelerpes, is associated 

 with tardiness in the development of these parts. 



