IV. VERTEBRATA: MAMMALIA. 621 



hyoid arch, the stapes (in part equalling the hyomandibular, st), 

 and the hyoid proper (h) (compare with the visceral skeleton of 

 the selachian, fig. 588). To these are added the membrane bones, 

 the squamosal (sq), at the base of the quadrate, which increases as 

 the latter loses in size, and below the squamosal the tympanic (ty). 

 With ossification of the cartilaginous parts several centres form 

 the petrosum, which fuses with the squamosal, and frequently 

 with the tympanic, which in some forms enlarges to a conspicuous 

 bulla ossea. Petrosum and squamosal on the one side, tympanic 

 on the other, enclose a space, the tympanic cavity, into which the 

 upper parts of both visceral arches extend, ossifying into the ear 

 bones, the quadrate to the incus, the hyomandibular possibly to 

 stapes (fig. 577). 



The tympanic in uniting with the squamosal (forming Glaser's 

 fissure) encroaches on the mandibular cartilage so that the upper 

 end (n), which is homologous with the articulare of other ver- 

 tebrates, is enclosed in the tympanic cavity and, along with a sec- 

 ond bone, the angulare, ossifies to form the malleus, while the 

 lower portion, Meckel's cartilage proper (mk), becomes pinched 

 off. Meckel's cartilage gradually disappears; on the other hand 

 the surrounding membrane bone, the dentary (de) increases and 

 alone forms the lower jaw, which now forms a new articulation 

 with the squamosal. It will be noticed that the old articulation 

 was between cartilage bones, the new between membrane bones 

 developing around the cartilages. (There is, however, some evi- 

 dence to show that the mammalian lower jaw consists of several 

 bones, some of them preformed in cartilage, and that one of these 

 forms the articulation with the squamosal.) 



The lower part of the hyoid arch, the hyoid, remains outside the 

 ear and often fuses with the petrosal. The upper end (styloid 

 process) may then become entirely separate from the lower, which 

 becomes attached to the copula (body of hyoid) as the anterior 

 horn, the connecting cartilage being reduced to a stylohyoid liga- 

 ment. In the hyoid of mammals there is also included a remnant 

 of a gill arch as the posterior horn. 



As the quadrate, by its modification into the incus, becomes 

 strikingly reduced, the rest of the arch vomer, palatine, and 

 pterygoid is poorly developed in contrast to the large maxillary 

 bones. Premaxillaries and maxillaries (fused in man to a single 

 bone) form an important element in the face, and send backwards 

 and inwards palatine processes into the roof of the mouth. These 

 encroach upon the bones of the palatal series; the vomers of the 



