196 



VERTEBRATE SKELETON 



exceptions (where parietal, ali- and orbitosphenoid meet), it aids in 

 enclosing, its participation being small in many Insectivores, bats 

 and some Marsupials. The lower part of the squama has a broad 

 oblique paramastoid process extending downwards and inwards 

 to the mastoid, while the zygomatic process (enormous in Sirenia 

 and large in many whales) usually meets the zygomatic. On the 

 lower side of the base of the process is the mandibular (glenoid) 

 fossa, a part of which in some mammals is formed by the zygomatic, 

 the alisphenoid contributing in some Marsupials. The form of the 

 fossa, in w^hich the lower jaw is hinged, depends on the motions of 

 the jaw. The fossa is often bounded behind by a postglenoid process 

 which adds to the strength of the articulation. 



The zygomatic arch is either the upper arcade of Diapsida or is derived from 

 the single arch of Synapsida by the loss of bones, and in all mammals it has lost 

 the quadratojugal (see, however, p. 182). In some it is nearly straight; others 

 have it stronger and curved outwards, the lesult of the great development of the 

 chewing muscles, but the arch is incomplete in many Insectivores where the 

 muscles are strong. In Monotremes the arch is composed of maxilla and squa- 

 mosal, the zygomatic being reduced in Ornithorhyiuinis. absent in Echidna. 



Fig. 205. — B, Postero- ventral part of skull of Paradoxurus; A , otic region of same with 

 tympanic removed; C, side view (Weber, '04); D, developing ear region of dog (van der 

 Klaauw, '22). b, otic bulla; bo, basioccipital; bs, basisphenoid; c, isolated cartilage; 

 en, condyle; ct, chorda tympani; e, developing entotympanic; /, facial nerve; h, hyoid; 

 i, incus; tn, external auditory meatus; ml, malleus; pa, postauditory process; pp, par- 

 occipital process; s, stapes; sq, squamosal; I, tympanic; tc, tympanic cavity. 



Closely associated with the petrosal is the tympanic bone which 

 begins ossification lateral to the anterior process of the malleus (pre- 

 articular part of Meckel's cartilage, figure 212); with growth it gradu- 

 ally curves downwards, below and then behind the auditory meatus, 

 forming a nearly complete ring lateral to the petrosal, a shape retained 

 in Monotremes, Marsupials, Sirenia, most Insectivores and many 

 Primates. In all cases it supports the tympanic membrane and 



