THE SKULL 



ior 



ovalis on the one hand and articulates with the incus on 'the 

 other, while the malleus articulates with the incus, and its 

 manubrial process is attached to the tympanic membrane. The 

 above facts indicate that the malleus corresponds to the articular 

 element of the mandible of lower Vertebrates, and the incus to 

 the quadrate. 



The morphology of the stapes is not by any means clear ; phylogenetically 

 it certainly corresponds to the upper end of the hyoid arch (pharyngo-hyal or 

 hyomandibular of Fishes), but its homology with this element has not been 

 proved ontogenetically. Its basal plate, however, doubtless corresponds to 

 the stapedial plate of Amphibia and Sauropsida (comp. pp. 84 and 92). 



e.n 



oc.e 



FIG. 80. SKULL OF EMBRYO OF ARMADILLO (Tatusia hybrida). (Modified from a 

 drawing by W. K. Parker. ) 



a.fy, 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 ; Ic, lachrymal ; mk, Meckel's 

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

 parietal ; jm, palatine ; px, premaxilla ; so, supraoccipital ; st, stapes ; s.t, 

 superior turbinal ; st.m, stapedius muscle ; $q, squamosal ; th, thyroid ; tr, 

 trachea ; F 1 , V 2 , foramina through which the first and second divisions of 

 the trigeminal pass out from the orbit ; //, optic foramen. 



Thus the parts of the mandibular arch which form the hinge of 

 the jaw in lower Vertebrates are in Mammals utilised to conduct 

 sound-vibrations to the internal ear. Around Meckel's cartilage 

 a membrane-bone is developed corresponding essentially to the 

 dentary ; this forms the entire lower jaw of the adult and develops 

 a new articulation with the squamosal : this arrangement is 

 characteristic of and confined to the Mammalia (Figs. 79 and 80). 

 The two rami of the mandible usually unite distally. 



Teeth, which are only exceptionally wanting (e.g., Echidna. 



