626 
¥. H. EDGEWORTH. 
strictor divides into dorsal and ventral portions^ and the 
anterior digastric becomes connected with the lower end of 
the ventral portion, e. g. Bradypus marinoratus, Choloe- 
piis, and probably the Marsupials examined by Kohlbrugge 
(vide p. 619). 
(2) The hyoid ventral constrictor divides into dorsal and 
ventral portions, and the anterior digastric becomes connected 
with the lower end of the dorsal portion ; the ventral portion 
forms, as a rule, the stylo-hyoid muscle. This condition is 
present in all Eutheria. Pliylogenetically, it is probably 
derived from sub-variety (b) above, rather than from sub- 
variety (a). Such a digastric muscle may be of many diffe- 
rent forms — classified by Bijvoet into digastricus verus 
and ^^digastricus spurius.’’ 
The posterior digastric of this latter class of Mammals is 
partially or wholly — according to whether a mastoideo- s. 
jugulo-hyoideus s. jugulo-branchialis is or is not developed — 
homologous with the dorsal portion of the posterior digastric 
of these Edentates, in which division into dorsal and ventral 
portions takes place. The mastoideo- s. jugulo-hyoideus pre- 
serves the ancient insertion of the muscle. The stylo-hyoideus 
is homologous with the ventral portion. It may pass to the 
middle line and join its fellow, thus preserving the primitive 
condition, as in Cynocephalus (Bijvoet) ; or, keeping the 
transverse aponeurosis it may gain an additional insertion to 
the branchial cornu, as in the pig ; more commonly, however, 
the aponeurosis disappears and the muscle is inserted into the 
basibranchial or its branchial cornu, as in the rabbit. The 
origin of the muscle often extends upwards on the stylo-hyal 
and may even reach the skull, so that it is overlapped by the 
posterior digastric, and the primitive positions of the two 
muscles are obscured. 
According to Bijvoet a stylo-hyoid muscle is absent in 
Erinacmus among Insectivora, the Cheiroptera, and some 
Mustelidm, but he also states that in some Erinacaeus some 
bundles of the posterior digastric stream to a thin membrane 
which passes inwards to the middle line ventral to the hinder 
