THE MANDIBULAR AND HYOID .MUSCLES OE 31AM:\[ALS. 029 
This identification affords an explanation of the statements 
of Huxley, Escliweiler and Fiirbringer, that a stapedius 
muscle, i.e. one of the usual mammalian type, is absent in 
Monotrernes, and also of the method of innervation. In non- 
Monotreme Mammals the nerve to the stapedius is given off 
from the N. facialis proxitmd to the chorda tympani. Now 
in Echidna — according to Schulman — the mastoideo-hyoideus 
is innervated by several fine twigs from the X. facialis, 
some of which are given off proximal to, and some with, the 
chorda tympani. 
The relations of the N. facialis to the levator hyoidei s. ^ 
stapedius in Echidna are different from those in Ornitho- 
rhynchus, Dasyurus, pig and rabbit. In the latter four 
animals the nerve passes backwards lateral to the muscle and 
then downwards. Earlier stages of Ornithorhynchns and 
Dasyurus were not available, but in the rabbit and pig this 
condition is preceded by one in which the nerve passes out- 
wards dorsal to the Anlage of the stapedius and hyoid ventral 
constrictor and then downwards. The levator hyoidei s. 
stapedius thus extends upwards on the inner, medial, side of 
the nerve to gain a dorsal attachment to the outer surface of 
auditory capsule or to the floor of the fossa stapedii. 
In stage 47 of Echidna the nerve passes backwards in the _ 
sulcus facialis dorsal to, and separated by the incurving crista 
parotica from the levator hyoidei Avhich arises from it. In 
stage 50 (figs. 36 and 37) the origin of the levator hyoidei 
has extended outwards to the base of the crista parotica, and 
in the adult (fig. 38) to the outer surface of the auditory 
capsule. The IST. facialis consequently passes backwards 
medial to the muscle, whereas in other Mammals it passes 
backwards lateral to it. 
It would thus appear that the name mastoideo-hyoideus^^ 
has been applied by various investigators to three muscles of 
differing origin and morphological nature : (1) to a stapedius 
muscle which has preserved a levator hyoidei stage, as in 
Monotrernes ; (2) to the dorsal portion of a hyoid ventral 
constrictor which has divided into dorsal and ventral portions, 
