ox THE REDUCTION OF THE JUGAL IX MAMMALIA. 71 



7. The Progressive Reduction of the JiiCTal in the Mammaliu. 

 By Lancelot T. Hogbex, B.A., B.Sc.* 



[Received Februiiry 27, 1919 ; Kead April H, 1919.1 



Althoiigli the chavfictei- of the zygomatic arch is at once a most 

 diagnostic feature of the Mammalia as a class, and one which 

 undergoes profound modification in the individual orders and 

 families, no adequate general account of it has been given since 

 (Slade t contributed to our knowledge of the features it exhibits 

 in response to the functions it is called upon to perform. 

 Significant as it is, however, to appreciate the manner in which 

 the development of the zygomatic arch depends on the energy 

 and character of the masticatory process, such treatment does not 

 exhaust the interest of its structure. Cases occur abundantly 

 where, though otherwise similar in curvature, resistance, and 

 functional significance, it is fundamentally dissimilar in the part 

 played by its component elements ; and a careful study indicates 

 a tendency for the suppression of the jugal element independently 

 manifested along totally divergent lines of mammalian descent. 

 Moreover, while the manner in which the zygomatic arch is 

 adapted to the requirements of difi'erent types of n)ammalian 

 organization leads to considerable diversity in its structure, there 

 are some striking particulars of resemblance where its physio- 

 logical importance may be totall}' different ; and the student of 

 morphology cannot fail to give consideration to any details 

 of contrast and comparison which do not seem to be called into 

 being by the conditions of environment and in consequence 

 capable of being attributed to convergent evolution. Cei-tain 

 points which remain therefore to be set forth or emphasized seem 

 to justify a brief survey of the I'ole played by the malar among 

 the mamuuilia, though it may necessitate repeating matter 

 already published. 



In no group of Placentals except the Carnivora does the 

 zygomatic arch show the same uniformity of structure as in 

 the Marsupials, and when the varied types of dentition and diet 

 exemplified by the latter group are taken into account, this fact 

 should merit serious attention in discussing the possible diphyletie 

 origin of the group. It may at least be inferred that the type of 

 structure which characterizes the Marsupials approximates closely 

 to the ancestral condition. In all living Metatheria the jugal 

 plays a conspicuous part in its formation — even in the singular 

 genus Noioryctes. It is a robust structiu-e usually curving out- 

 wards conspicuously. The relations of the jugal are in general 

 the same : it extends from the la.crymal antero-dorsally at the 

 border of the orbit to the glenoid postero-ventrally. The 



* Comuuiuicsitcd hy Mr. H. W. Unthank, 1>.A., B.Sc, P\Z.S. 

 t Slade, D. I). "The siguifieaiice of the Jugal Arch." Fr. Amer. Phil. Soc. 

 xxxiv., 1895. — Cf. also Hallmau, E., "Die Vergl. Osteol. die Sclilateuljeiiis." 



