THE MANDIBULAR AND HYOID MUSCLES OF MA:\IMALS. 009 
the genio-hyoid retains its attachment to the basibranchial, 
and in Didelpliys anrita the hyoglossus is attached to the 
first branchial bar. In Echidna/ where there is no mammary 
teat, there is, in stage 50, a complicated system of protractors 
and retractors of tlie tongue, the former consisting of genio- 
glossLis, genio-glossus posticus externns and internns, the 
latter of sterno-glossus and laryngo-glossus (figs. 34-37). 
The sterno-glossus passes through a loop formed by a M . 
annulus. The transverse lingual fibres are not specially 
developed. The external pterygoid and depressor mandibula) 
anterior are developed. The posterior end of the genio-hyoid 
is attached both to the first branchial cornu and to the thyroid 
cartilage. A sterno-thyroid is attached anteriorl}" to the 
posterior edge of the thyroid cartilage. The omo-hyoid, 
arising from the medial surface of the scapula, is in part 
inserted into the thyroid cartilage and in part joins the 
genio-hyoid. 
’•On comparison of the above phenomena it would appear 
that there are two common characteristics of the jaw-muscles 
in Marsupials and Monotremes which may be supposed to 
have played a part in the phylogenetic change of an incudo- 
meckelian to a squamoso-mandibular joint: (1) The develop- 
ment of an external pterygoid. (2) The development of a 
(transverse) depressor mandibulae anterior. This muscle in 
Dasyurus quickly becomes oblique and forms the anterior 
digastric. The condition in Monotremes, however, negatives 
any idea that a digastric had any share in the phylogenetic 
development of a squamoso-mandibular joint. In Echidna 
and Marsupials various secondary connections are developed 
between the elements of the hypo-branchial spinal muscles 
which are situated behind the first branchial bar (i. e. sterno- 
hyoid, omo-hyoid, and thyro-hyoid) and those in front (i.e. 
^ This account differs from that given by Fewkes in (1) the more 
extended posterior insertion of the genio-hyoid, (2) the existence of a 
sterno-thyroid, (3) the existence of an omo-hyoid — confirming the 
statement of Leche. The M. annulus was not yet separated into 
M. annulus inferior and M. annulus intimus. 
