REDUCTION OF THE JUGAL IN MAMMALIA. 77 



meet the alisphenoicl, is to be found in the fact that the jugal 

 articulates witli the laciymal in the former ))ut not in the lattei' 

 group. It is frequently stated, even by such authorities as Weber*, 

 that the Catarrhina are distinguishal)le from the Plnt}rrhina by 

 the junction of the jugal with the pnrietnl in the last named : but 

 this is not by any means a universal rule. In Mycetes the jugal 

 does not meet the parietal ; and a specimen of Ateles ater in the 

 Birkbeck College collection shows the jugal joining the parietal 

 on one side of the skull but not on the other. 



A comparison of the skull in the Anthropoidea with that of 

 forms like Chiromys and Indris suggests that even in the 

 Primates, in which the role of the jugal has become relatively 

 more important on account of its inclusion in the orbit, it has not 

 escaped the general tendency towards reduction by the invasion 

 of the squamosal or maxilla or both apparently developed in- 

 dependently along the various lines of mammalian phylogeny, as 

 indicated in the previous survey. It is -without doubt true that 

 so far as the curvature and strength of the zygomatic arch is 

 concerned, its development, as iSlade observes, "depends upon the 

 enei'gy and character of the masticatory process." Nevertheless, if 

 the problem is approached in the light of modern evolutionaiy 

 theory with the mind less taxed with the necessity of discovering 

 utility in every sti'ucture, it is difficult to reveal any general 

 teleological significance in the part individually contributed hj 

 the various elements of which it is comjjosed. Seeing that in a 

 diversity of isolated genera among the Placentals exhibiting every 

 possible variety of diet and habit, and also in some of the less 

 specialized representatives of the larger groups themselves, the 

 jugal displays essentially the same relations as in the Metatheria, 

 namely, extending postero-ventrally from the glenoid to the 

 lacrymal antero-dorsally, it is hardly possible to agree with Weber 

 that the jugal was, small in the earliest Mammalia (op. cit.) as in 

 the Insectivora of to-day : on the conti^ary, there can be little 

 doubt that this represents the ancestral condition retained by the 

 class till a date later than that at which the modern lines of 

 Mammalian descent had become differentiated. In some cases the 

 jugal has become effectively eliminated although the arch is 

 eomplete and even robust, as in the Monotremes f where the 

 malar is vestigial or absent, at least as a separate ossification. 

 The reason for this progressive reduction of the jugal quite 

 irrespective of the form a.nd function of the arch, as is most 

 emphatically demonstrated by a comparison, for instance, of the 

 skulls of Erethizon and HyclrochcpriijS, remains obscure. Neverthe- 

 less, the manner in which it is efieeted among the different orders 

 of MammaKa is so characteristic and, in a sense, conservative that 

 similarity in the morphological relations of the jugal in Mammals 



* Weber, Max. Einfiihriuig in die Anatoniie unci Systematik der recenten und 

 fossilen Mammalia. .Jena, 1904. 



f Vide Sixta : '' Untevsuclinng iiber den Ban des Scliaedels von Monotremen und 

 Reptilien," Zeitschr. f. Morph. und Antbropol. ii.. 1900. 



