134 COMPARATIVE ANATOMY 



between the fenestra ovalis and the tympanic membrane, and 

 serving to conduct sound vibrations to the inner ear. 1 An investing 

 bone, the dentary, is developed around the main part of Meckel's 

 cartilage, distal to the malleus ; the cartilage itself may undergo 

 partial ossification, but gradually disappears, the dentary forming 

 the bony mandible, which develops a new articulation with the 



e.n 



FIG. 95. SKULL OF EMBRYO OF ARMADILLO ( Tatusia hybrida). (Modified from 

 a drawing by W. K. Parker. ) 



a.ty, tympanic annulus ; au, auditory capsule ; b.hy, basihyal ; c.hy, ceratohyal ; 

 cr, cricoid ; d, dentary ; e.hy, epihyal ; e.n, external nostril ; eo, exoccipital : 

 f, frontal ; h.hy, hypohyal ; i. jugal ; in, incus; lc, lacrymal ; mk, Meckel's 

 cartilage ; ml, malleus ; mx, maxilla ; n, nasal ; oc. c, occipital condyle ; 

 p, parietal ; pa, palatine ; px, premaxilla ; so, supraoccipital ; st, stapes ; 

 s.t, ethmoturbinal ; st.m, stapedius muscle; sq, squamosal ; th, thyroid; 

 tr, trachea ; //, optic foramen ; V 1 , V 2 , foramina through which the first 

 and second divisions of the trigeminal pass out from the orbit. 



squamosal, characteristic of, and confined to, the Mammalia, all 

 other Craniata possessing the more primitive quadrato-mandibular 

 articulation. The two rami of the lower jaw may remain distinct 

 at the symphysis, or many unite with one another (e.g. Bats, 



1 There is some doubt as to how far it is justifiable to consider the tympano- 

 eustachian cavity as homologous with the spiracle of Fishes, and the tympanic 

 cavity and membrane of Amphibia, Sauropsida, and Mammalia as homologous 

 with one another. 



The stirrup form of the stapes is due to its being perforated by an artery 

 (as in the case of the stapedial plate of the Gymnophiona), which in 

 certain cases persists in the adult. The stapes, however, is not perforated in 

 Monotremes and certain Marsupials and Edentates. The homology of this 

 element is by no means clear, but there are reasons for considering it to correspond 

 to the stapedial plate of the Sauropsida and to the whole columella of Amphibia ; 

 it is possible that all these structures are derivatives of the hyomandibular of 

 Fishes. 



