The Columella Aiiris in Amphibia. 577 



In the fenestra vestibuli of the adult there is a single plate, irregu- 

 larly oval in outline and free from the ear capsule except on its 

 ventro-cephalic margin where a fusion is found (PI. X, Fig. 67). 

 From the cephalic and dorsal portion of this plate there projects 

 upwards and forwards a slender stilus (Fig. 67) which in the adult 

 articulates with both quadrate and squamosum. It is slender in all 

 forms except Batracoseps where it is absent or vestigial. In the 

 caudal portion, the fenestral plate extends behind the caudal margin 

 of the fenestra and swells outward, forming a prominence in this 

 region. The cavity of this prominence is an outward and backward 

 extension of the cavum perilymphaticum. In a relatively deep de- 

 pression on the caudo-Iateral aspect of this prominence the M. oper- 

 cularis is attached. The fenestral plate in the Plethodontidse is, 

 as are all the related parts, finer and much less massive than in the 

 families already considered. The stilus is relatively a long and 

 slender rod between the vena petroso-lateralis above and the arteria 

 carotis interna and facial nerve below. The lips of the fenestra 

 vestibuli remain cartilaginous and are connected with the fenestral 

 ])late by membrane only, save in the cephalo-ventral portion mentioned 

 above. 



The inner and outer bony plates so characteristic of the columella 

 in other forms are here co-ossified, the cartilage persisting only at the 

 circumference. In the central portion of the plate the cartilage 

 which is subsequently replaced by bone, varies marhedly in thick- 

 ness even in the same genus, — for example, in Spelerpes bislineatus 

 it is small in amount and early replaced by bone, while in Sperlerpes 

 ruber quite the reverse is true. A similar comparison could be made 

 between genera. The ossification of the fenestral structures in this 

 family corresponds to the ossification of the skull as a whole, both 

 in time and extent. 



Accompanying this marked ossification of the plate there is com- 

 plete ossification of the base of the stilus ; distally it is composed 

 of a shell of bone enclosing a cartilaginous core. In regard to the 

 relation of the peripheral end of the stilus, it should be stated that 

 the articulation Avith the squamosum, quadrate and palatoquadrate 

 mentioned above applies only to the adult. In the larvae of the forms 



