THE MANDIBULAR AND HYOID MUSCLES OF .MAMMALS. 625 
bar. The geuio-hyoid and hyo-glossus arose from the basi- 
branchial and first branchial bar and extended forward to the 
anterior end of the lower jaw and tongue respectively. A 
branchio-hyoid s. cerato-hyoid muscle, between the first 
branchial and hyoid bars, was also present. The posterioi- 
digastric consisted of upper and lower parts, the upper 
extending from the mastoid to the hyoid bar, the lower from 
the hyoid bar to an aponeurosis immediately behind and con- 
tinuous with the intermaudibularis. A stylo-glossus arose 
from the hyoid bar. 
In this specimen, then, there was evidence of fusion between 
a sterno-hyoid and anterior digastric to form a sterno-man- 
dibularis. In the other Edentates possessing a sterno- 
mandibularis Toldt does not state whether the muscle was 
investigated microscopically. 
These phenomena tend to show that the connection of^the 
sterno-hyoid to the transverse aponeurosis of the posterior 
digastric is a secondary phenomenon and related to its non- 
attachment to the first branchial bar, and suggest that its 
fusion with the anterior digastric to form a sterno-mandi- 
bularis is also related to the same occurrence. 
The theory of Toldt, again, quite fails to account for the 
fact that in Dasyurus the anterior digastric, when first formed, 
grows out transversely just as in Ornithorhynchus, and only 
subsequently takes up a longitudinal direction. 
The theory of Toldt is thus open to many objections, and 
may probably be rejected in favour of that of Schulman and 
Bijvoet. 
The above observations, however, show that there are two 
main varieties of a digastric muscle in mammals. 
(1) The anterior digastric becomes connected with the 
ventral end of the hyoid ventral constrictor s. stylo-hyoideus ; 
no stylo-hyoid muscle is present. This occurs in Marsupials 
and those Edentates in which a digastric is formed. There 
are two sub-varieties: (a) the hyoid ventral constrictor 
remains undivided in most Marsupials and some Edentates, 
e.g. Bradypus tridactylus; (b) the hyoid ventral con- 
