The Columella Aiiris in Amphibia. 



579 



Despite the depression for the M. opercularis the ear capsule has 

 in this region a bulging appearance clue to the growth of the fenes- 

 tral plate backwards beyond the margin of the fenestra vestibuli. 

 Fig. 47 (PL VI) shows not only that the prominence has extended 

 beyond the margin of the fenestra, but also that within the promi- 

 nence there is a recessus w^hich is continuous with the cavum peri- 

 lymphaticum. 



Earlier in this paper it has been shown that in Ambystoma and 

 others there is a perilymphatic prominence in the region of the 



Figs. G and 7. Pletliodou ciuereus, embryo. C, arteria carotis interna; 

 O.I., canalis lateralis; Col., columella; H., ceratohyale; L.h-e., ligamentum 

 hyo-columellare ; M., musculus cephalo-dorso-mandibnlaris. 8p., os squa- 

 mosum ; V.p-l., vena petroso-lateralis. 



fenestra vestibuli formed by the outward and backward growth of 

 the operculum and that to the lateral aspect of this structure the M, 

 opercularis is attached. Within the perilymphatic prominence is 

 the recessus perilymphaticus, a caudal continuation of the peri- 

 lymphatic cavity. In these respects there is a striking similarity 

 between the caudal portion of the fenestral plate in the Plethodon- 

 tidse and the operculum of other forms. Further than the similari- 

 ties mentioned in the preceding paragraph there is, in the adult, no 

 evidence as to the constitution of the fenestral plate in the Pletho- 

 dontidse. In the light of development, however, the condition is 



