SKULL — AMPHIBLA. 



IIQ 



in most Tetrapoda, and as most of the species live in the air, the 

 necessity of transmission of sound waves across the middle ear (tym- 

 panum) has resulted in the development of a series of ear-bones 

 (ossicula auditus) which has an extensive literature. A number of 

 points regarding these are unsettled, but the following statement 

 gives the conclusions of those who have attacked the problems from 

 the ontogenetic and comparative standpoint. Many paleontologists 

 hold views differing in several respects. 



All Tetrapoda have an opening (fenestra vestibuli seu ovale, 

 in the lateral wall of the otic capsule, in which is a skeletal structure 

 (stapes), preformed in cartilage, (called also 'operculum,' although 

 two other bones have received this name) . A process (stylus) extends 

 laterally from this basal plate and 

 articulates with one (Columella, 

 Amphibia, Sauropsida) or with 

 the first of two (incus, malleus, 

 Mammals) more distal elements, 

 the series in either case, ending 

 distally in the tympanic mem- 

 brane. In Urodeles and Gymno- 

 phiona where no tympanic cavity 

 is developed the ossicula are not 

 connected with a tympanic mem- 

 brane, but in some genera (fig. 

 131) the stylus articulates with the quadrate.^ In all except these 

 and the mammals, the distal element is the extracolumella or hyo- 

 stapes; several parts are distinguished in it. The mammahan ossi- 

 cula auditus will be described in connexion with that group. 



The homology of the stapes is uncertain, but the mass of evidence — its posi- 

 tion posterior to the seventh nerve, its formation close to, but independent of 

 the otic capsule, and its chondrification from the same blastema as the cera- 

 tohyal — goes to show that it is the homologue of the piscine hyomandibula, an 

 element otherwise lacking in Tetrapoda. 



AMPHIBIA. — The skull of the lowest Amphibia, the extinct 

 Stegocephala, is very primitive and was used as the basis of the 



'.\pparently Urodeles have two distinct plates in the vestibular fenestra, one, the 

 stapes, formed lateral to and independent of the otic capsule, from the same stroma 

 which forms the hyoid arch. The other arises posterior to this by segregation from 

 the capsule itself. 



Fig. 125. — Diagram of ossicula auditus 

 of (.4) mammal and {B) Sauropsidan. 

 c, columella; e. Eustachian tube;/?/, fen- 

 estra vestibuli ;/, incus ; m, body of malleus ; 

 pi, internal process of columella; s, stapes 

 and stylus; tc, tympanic cavity; Itn, tym- 

 panic membrane. 



