OSTEOLOGY. 291 
If this interpretation be allowed, it is evident that the so-called ‘‘squamo- 
sal” is a greatly expanded jugal that has become flattened and appressed against 
the wall of the brain-case so as to obliterate almost completely the temporal 
fossa, of which the temporal canal for the passage of the external carotid is 
the only vestige. In Ornithorhynchus the posterior portion only of the jugal 
is applied to the brain-case so that the canal is very short and the orbitotempo- 
ral fossa nearly open. Further evidence is afforded by the muscles, for the 
temporalis, which inserts usually on the jugal, is inserted on the zygomatic 
portion of this bone, and the digastric, which usually originates anteriorly from 
the mastoid region, is in living monotremes found arising from what is here 
considered the true mastoid or squamosomastoid, not from what van Bemmelen 
considers the squamosal. Against this hypothesis is the belief of van Bemmelen 
that the process on the anterior dorsal edge of the arch in Ornithorhynchus is 
a true processus frontalis, and he further adds that in a young skull he found 
this process separated by a slight suture from the processus jugalis squamosi. 
In a second young skull, however, he found no such condition. It seems emi- 
nently probable that this process in Ornithorhynchus is not the homologue of 
the frontal process in higher mammals, but a dorsal extension of the maxil- 
lary portion of the zygoma. Perhaps a more serious objection to the above 
interpretation is that the glenoid cavity is entirely lined by an inward extension 
of the jugal. That the jugal should share in the formation of the articulating 
surface, however, need prove no difficulty, for such a condition obtains, though 
in much less degree, among the Marsupialia. Thus in the Giant kangaroo 
(Macropus giganteus) the posterior extension of the well-developed jugal lies 
against the ventral border of the squamosal process and actually forms part of 
the articulating surface for the broad condyle of the jaw. In other marsupials 
(e. g. Didelphys) this extension merely reaches the glenoid cavity and forms 
its lateral boundary. It is not difficult to conceive of its extension to cover 
the floor of this cavity as it appears to do in Proechidna. Moreover, Gaupp 
(1908) has figured this bone in a partial reconstruction of the primordial cranium 
in the embryo Echidna, and at this stage apparently, there is no inward exten- 
sion to the glenoid cavity; the bone seems too far anterior to the mastoid region 
to fulfil the requirements of a squamosal. It is further of interest to note the 
tendency to a posterior dorsal expansion of the jugal in certain other marsupials 
so that with the ventral extension a V is formed into which the squamosal 
process fits. This dorsal arm is probably the homologue of the broad scale- 
like expansion which in the Proechidna covers the lateral wall of the brain-case, 
