72 ^ MR. L. T. HOGBEN ON THE 



squamosal, liowever, always occupies a large proportion of the 

 dorsal side of the arch ; and it is perhaps not strictly accurate to 

 state* that the jugal always actually participates in the formation 

 of the glenoid cavity, because though the latter may be compact, 

 as in Fhascolarctus, its limits ai'e not always cleni'ly defined. Not 

 infrequently it is a large smooth area, tapering off imperceptibly 

 to the flattened ventral rim of the arch, and when the condyle of 

 the mandible is small only a portion of this surface is strictly 

 articular. It is certain for example that the head of the mandible 

 in its normal rotation does not come into contact with the jugal 

 in the Rat Kangaroo, Hyj^sijirymiius rufescens. The postorbital 

 process of the zygomatic arch is but slightly indicated, indifferently 

 by the squamosal or jugal or both among closely allied genera, and 

 the same variability in the relation of the different elements of the 

 zygomatic arch to the eye-socket is encountered ^mong the various 

 orders of the Placentals. Widely as the structure of the zygo- 

 matic ai'ch differs among the Placentalia, tl>e part played by the 

 jugal in its composition varies more remarkably. lu a large 

 number of genera that occupy a somewha,t isolated position, and 

 appear to have diverged from the main linQ<s of mammalian 

 phylogeny at an early date, and also in some cases in the less 

 specialized members of the lai-gei- groups, the jugal displays Avhat 

 may be regarded as the ancestral condition, extending from the 

 lacrymal antero-dorsally to the glenoid postei^o-ventrally. But 

 more generally it becomes displaced by the encroachment of the 

 squamosal and maxilla, its reduction being sometimes accompanied 

 by a sti-engtheiiing of the zygomatic arch or in others by a 

 weakening, and frequently without any evident modification 

 ■ either of its contour or its rigidit}^. 



In the Rodentia the zygomatic arch is alwa3^s comparatively 

 well developed ; and no student of this group can fail to be 

 impressed by the highly characteristic modifications it undergoes 

 among the various families. Brandt t, following Waterhouse, 

 paid particular attention to the character of the zygomatic arch 

 in his classification of the Rodents ; and it may be said that liis 

 system would have approximated more nearly to those of sub- 

 sequent investigators +, if he ha,d studied the rela,tion of its 

 constituent elements in more detaH. Thus the Bathyergidfe 

 ^jlaced by him with the Myomorpha, the Anomaluridas and 

 Haplodontidte in the Sciuromorpha,, should be separated from 

 those groups on account of the relations of the jugal bone if for 

 no other reason. 



* Weber, Max: "es erstrcckt sicli bis zur Fossa gleuoidea ixiid biUlet deien 

 Ausse.iiflaclie." Eiiifiihrung in die Anatomie systeiiiatik der rccenteii uiul fossilen 

 Mammalia. 



f Brandt, J. F. " Untei'sucliungen iiber die craniologiscben Eutwickelinigsstufen 

 iind die davon beiziileiteten Verwaiidtschaften und Classificationen devNager der 

 Jetztzeit, mit besonderer Beziebung auf die Gattung Castor." Mem. Ac. Sci. St. 

 P6tersbourg, 1855. 



X Winge, H. " Jordfundne og iiulevende Gnavere." E Museo Lundi, 1888. 

 Tullberg, T. " Ueber das System der Nagethiere." Upsala, 1899. 



