86 FRANKLIN PEARCE REAGAN 



lated with the brain-case. Here the palatoquadrate is firmly 

 fused with the cranial wall, and the upper end of the hyoid arch 

 (hyomandibula) takes no part in this union; this is a distinct 

 advance towards the differentiation of a definite quadrate. 



The skeletal complex of the first visceral arch forms the car- 

 tilage of the jaws. The antero-dorsal cartilage is known as 

 pterygoquadrate. The ventral element comprises Meckel's 

 cartilage. In selachians these two are suspended from the skull 

 by the hyomandibular cartilage, the latter having been derived 

 from the dorsal portion of the second visceral arch. During 

 the ontogeny of the higher vertebrates the pterygoquadrate and 

 hyomandibular homologues become very closely associated with 

 the developing cranial skeleton, either losing entirely their 

 identities, or becoming greatly transformed. Certain it is that 

 the jaw articulations of the lower forms are not homologous with 

 those of higher ones. 



The hyomandibular, or at least the dorsal portion of the 

 second visceral cartilage, is believed by many to be represented 

 by that columnar bone in the series of ear-ossicles which be- 

 comes the most intimately associated with the otic capsule. In 

 amphibia, sauropsida, and monotreme-mammalia, this homo- 

 logue has been called by various names such as 'columella,' 

 'columella auris,' and 'columella cranii.' The cartilaginous 

 plate fitting into the fenestra ovalis has been designated in 

 amphibia as the 'operculum.' In higher forms the cartilaginous 

 plate occupying the fenestra ovalis is known as the stapedial 

 plate. 



The terms 'columella' and 'columella cranii' have been em- 

 ployed with a great deal of confusion, as, for instance in the 

 writings of Gegenbaur and Schimkewitsch. Gegenbaur (Vergl. 

 Anat. der Wirb., p. 374) interprets the columella of amphibia 

 as follows: 



In iirodeles there is a ligamentous process stretching from the oper- 

 culum to the cartilaginous quadrate. In the anura the operculum is 

 continued as an elongate ossified staff, the columella, which is to be 

 regarded as a part of the auditory apparatus. These are two skeletal 

 units which have taken the place of the hyomandibulare. 



