The Columella Auris in Amphibia. 



583 



amen which increases in size with the growth of the skull. The early 

 ossification of the ear capsule renders impossible the formation of 

 an operculum by either the cutting out process or by forward growth 

 into the fenestral membrane. A comparison of Figs. 11 and 12 will 

 help to make clear that the membrane filling the caudal portion of 

 the fenestra corresponds in its position and relation to other parts 

 to that portion of the fenestra in Ambystoma which is occupied by 

 the operculum. This, together with the formation of a prominence 

 by the fenestral membrane in this, region, which when chondrified 

 affords attachment for the M. opercularis, suggests that it is oper- 

 cular tissue. Taking this view it is perhaps not inappropriate to 

 consider that the caudal portion of the fenestral plate in the Pletho- 

 dontidse represents the operculum of other forms. 



Fig. 11. Ambystoma punctatum, transforming larva. G.I., canalis later- 

 alis ; C.p., cavum perilympbaticmn ; D.p., ductus perilymphaticus. M.. musculus 

 cephalo-dorso-mandibularis ; Op., operculum. 



Fig. 12. Spelerpes bislineatus, larva, 55 mm. long. Lettering as in Fig. 11. 



This is from the viewpoint of groups in which the operculum is 

 formed as a separate structure. Looking at it from the reverse 

 point of view the Plethodontidse might be considered as representing 

 the less specialized condition. It is conceivable that in those forms 

 in which an operculum is developed the columellar blastema became 

 fused with the edge of the primitive fenestra both in front and be- 

 hind, so that when it came to cliondrification there were almost of 

 necessity, developed two plates: (1) the fenestral plate of the col- 

 umella joined to the otic capsule in front, and (2) the operculum 

 joined to the ear capsule on the caudal and medial side. The oper- 

 culum would thus be a dissociated part of the fenestral plate. In 



