606 
F. H. EDGEWORTH. 
to the differentiation of the individual muscles which 
characterise Mammals. Similarly, the veutro-median group 
probably dates from a period antecedent to the differentiation 
of a depressor mandibulae anterior s. anterior digastric from 
the intermandibularis. 
The buccal nerve is generally described as a branch of the 
ramus lateralis, alongside of which it runs for a short distance 
in the adult. It would be preferable, however, in view of its 
earlier development and differeut function, to describe it as a 
separate branch of the mandibular division of the fifth nerve, 
and to restrict the term ramus lateralis ” to the muscular 
branch. 
The intermandibularis in early stages of Ornitho- 
rhynchus, Dasynrus, rabbit and pig forms a ventrally curved 
muscular sheet, with a median raphe, and attached laterally 
to the inner surface of MeckeFs cartilage. At a late stage, 
in Dasyurus, pig and rabbit, it becomes attached to the 
mandible. In Ornithorhynchus (Schulman) the muscle is 
attached laterally to the lower jaw, ligamentum pterygo- 
mandibulare, os pterygoideum and annulus tympanicus, the 
last-mentioned portion forming a separate muscle, the 
tympanico-hyoideus. In Echidna, stage 47, the muscle has 
already extended backward, but its anterior part is still 
attached to MeckeTs cartilage ; in stage 50 this attachment is 
lost and the fibres extend up towards the outer edge of the 
(Monotreme) pterygoid bone ; the fibres behind are attached 
to the tympanic bone. In the adult (Schulman) the muscle is 
attached to a fascial sheet at the level of the foramen 
rotundum, the hinder half of the outer edge of the os palatiuum, 
the os pterygoideum, the ligamentum pterygo-mandibulare, the 
tympanicum and the stylohyal. The above mentioned embryo- 
logical phenomena support liis view that the absence of attach- 
ment of the muscle to the jaw is a secondary occurrence. 
On the Homologies between the Masticatory 
Muscles of Mammals and Non-mammals. — The embryo- 
logical evidence detailed above suggests that Mammals have 
originated from forms characterised by two masticatory 
