MORPHOLOGY OF THE VERTEBRATE SKULL 19 
noid process and mandibular condyle is in all probability homol- 
ogous with the ‘ascending process’ of the reptilian dentary. This 
ascending ramus was originally a process for the insertion of 
muscles. It became differentiated into two parts, an antero- 
dorsal branch (coronoid) for the temporal muscle and a postero- 
inferior branch (condylar process) for the external pterygoid 
muscle. 
2. The joint between the lower jaw and the skull in mammals 
is solely a squamoso-dentary joint which arose in front of the 
old quadrato-articular joint and in which neither of the old ele- 
ments (quadrate, articular) participate. In contrast with most 
Fig. 16 Scheme showing approximate relations of lower jaw to skull in Cyn- 
ognathus. 
m.a.e. auditory groove. 
other joints, the squamoso-dentary joint is thus a secondary 
attachment of two elements which formerly had no connection. 
This is also indicated in the ontogeny (see paragraph 6 below). 
3. The quadrate and articular elements of the old reptilian 
jaw articulation no longer function as such in the mammalian 
skull but have become transformed into accessory auditory 
ossicles (incus, malleus, see p. 24). 
4. Thus the special peculiarity of the mammalian as compared 
with the reptilian lower jaw is that in the mammals the anterior 
tooth-bearing element has become completely separated from the 
posterior half of the jaw and the two halves now subserve two 
widely different functions (mastication, audition). 
