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 abiiost completely the temporal 

 fossa, of which the temporal canal for the passage of the external carotid is 

 the only ^'estige. In Ornithorhjaichus 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 li\'ing 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 hned 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 

 {Macro pus 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 prunordial 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 mterest to note the 

 tendency to a posterior dorsal expansion of the jugal m certam 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, 



