IV. VERTEBRATA: MAMMALIA 549 



ing into the ear bones, the quadrate to the incus, the hyomandibular pos- 

 sibly to stapes (fig. 534). 



The tympanic in uniting with the squamosal encroaches on the mandib- 

 ular cartilage so that the upper end (n), which is homologous with the 

 articulare of other vertebrates, is enclosed in the tympanic cavity and, 

 along with a second bone, the goniale, ossifies to form the malleus, 

 while the lower portion, Meckel's cartilage proper (rnk), is pinched off, to 

 form the axis of the lower jaw. Meckel's cartilage gradually disappears; 

 on the other hand the surrounding membrane bone, the dentary (de) in- 

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

 with the squamosal. It will Ije noticed that the old articulation was 

 between cartilage bones, the new between memijrane bones developing 

 around the cartilages. (There is, however, some evidence to show that 

 the mammalian lower jaw consists of several bones, some of them pre- 

 formed in cartilage, and that one of these forms the articulation with the 

 scjuamosal.) 



The transformation of the articulare and quadrate of the lower vertebrates 

 into the malleus and incus and the consequent view that the articulation of the 

 jaws is not homologous is strongly opposed by some, who see difficulty in the 

 transfer of the hinge. It must be remembered that the transfer is necessary 

 upon any hypothesis, and loses some of its difficulties, if we suppose a time 

 when both articulations were functional. 



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

 and often fuses with the petrosal. Its 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 ligament. 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 on either side) form an im- 

 portant 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 two sides are pressed together to a 

 single bone lying vertically entirely within the nasal partition ; the palatine 

 and pterygoid are forced backwards. The palatines contribute to the 

 hard palate, the pterygoids only exceptionally (Cetacea, many edentates) ; 

 the latter usually lose their independence and fuse vnth the nearest bone 

 of the base of the cranium, the basisphenoid (more accurately with a 

 process of the basisphenoid, the lamina externa of the pterygoid process, 



