ORIGIN OF THE LAND VERTEBRATES 105 



auditory ossicles, which are attached by one end to 

 the ear-drum and by the other to the internal ear. 

 Now this middle ear cavity is developed in the land 

 Vertebrate from the spiracular gill-cleft which no 



Fig. 22. A. Hyostylic skull of a Selachian, e.g. a Dogfish, in which 

 the upper jaw or palato-pterygo-quadrate bar p.p.q. is attached 

 loosely to the cranium by means of the ethmo-palatine ligament 

 e.p. and by ligaments attaching it to the hyomandibular carti- 

 lage hm. au. auditory capsule; hy. ceratohyal cartilage; l.j. 

 lower jaw ; n. nasal capsule ; o. orbit. B. Autostylic skull of a 

 terrestrial Vertebrate, e.g. a Frog, in which the upper jaw, com- 

 posed of the quadrate, palatine and pterygoid, is firmly fused 

 with the skull. The hyomandibular cartilage has been trans- 

 formed into the columella auris (col.) which conveys the sound- 

 waves from the tympanum t. across the middle ear to the internal 

 ear situated in the auditory capsule au. 



longer breaks through to the exterior but is closed 

 by a membrane which now forms the tympanum or 

 drum of the ear. Internally the middle ear opens 

 into the throat by the Eustachian tube, which is the 



