608 
F. E. EDGEWOllTil. 
Ampliibia, and a distinct homology exists between the medial 
and lateral masticatory muscles in the two Vertebrate groups. 
In Mammals these two muscles undergo considerable cliauges 
as detailed above, so that no homology exists between au}' 
one Mammalian masticatory muscle and any one Amphibian 
muscle. 
The intermandibularis is homologous with the similarly 
named muscle in other Vertebrate groups, Elasmobranchs, 
Teleostomi, Amphibia and Sauropsida. Its development in 
mammals, just as in other Vertebrates, lends no support to the 
theory of Huge that it is primarily a hyoid muscle. 
As will be shown later (pp. 579, 613, 616), the depressor 
mandibularis anterior of Ornithorhynchus and the anterior 
digastric of Dasyurus, rabbit and pig, are formed by pro- 
liferation from the ventral surface of the intermandibularis. 
There are no homologies in other Vertebrate groups, so that 
the muscle must have arisen within the Mammalian phylum. 
On the Changes in the Jaw Muscles accompany- 
ing t h e D e v e 1 o p m e n t of the S q u a m o s o - m a n d i b u 1 a 1 - 
Joint in Mammals. — Dasyurus and Echidna are born at an 
early stage with a functional incudo-meckelian joint, and the 
change of jaw-joint occurs during the pouch stage of existence. 
In the earlier stages of Dasyurus the prominent feature is 
the great development of the lingual muscles — the genio- 
glossus, hyo-glossns, stylo-glossus, and transverse lingual 
muscle-fibres ; this and the concavity of the anterior part of 
the dorsal surface of the tongue are intimately associated with 
the intrabuccal position of the maternal teat. Then follows the 
development of two new muscles — the transversely directed 
anterior digastric, which soon, however, becomes more oblique 
in position, with its medial end attached to the transverse 
aponeurosis of the hyoid ventral constrictor, and the external 
])terygoid. The adjacent ends of the genio-hyoid, sterno- 
hyoid and omo-hyoid lose their attachment to the first branchial 
cartilage and become united by tendon. These muscle phe- 
nomena precede and apparently initiate the development of 
the sqiiamoso-maudibular joint. In Phascolarctos, how^ever. 
