REDUCTION OF THE JUGAL IN MAMMALIA. 73 



III the Sciui'icUe and Castoridiie the jagal is least reduced, 

 foi-ming ahnost the entire arch and extending from the lacrynial 

 antero-dorsally to participate in the formation of the glenoid 

 postero-ventrally. In most of the remaining families the jugal is 

 invaded by the maxilla from before and the squamosal from behind. 

 In a number of families the jugal still meets the laciymal : such are 

 the Pedetidse and i^nomalurida?, Dipodidte, Bathyergidpe, and, in 

 marked contrast with the other Hystricomorphine families, the 

 Chinchillida-. In the Hystricidpe, Erethizontida?, and the Caviidaj 

 the zygomatic process of the maxilla encroaches further and 

 occupies a large portion of the arch excluding the junction of the 

 jugal vvitli the lacrymal ; but the jugal nevertheless takes part in 

 the formation of the glenoid cavity, although in the larger Oavies 

 it is covered over on the external side of the nrch by a superficial 

 extension downwards of the squamosal. This is conspicuously the 

 case in Hyilrochoerus, in which genus the jugal appears on the 

 outer side to be reduced to a narrow vertical wedge of bone 

 between the zygomatic processes of the maxilla and squamosal, 

 albeit in curvature and general cliaracteristics the zygomatic arch 

 conforms in a striking manner to the l)ystricine type. In fact, 

 HydrochcerKjS, which in several respects apart from its great size 

 appears to be one of the most highly specialized members not only 

 of the Caviidaj but of the Rodents, illustrates forcibly the general 

 tendency among the Mammalia for the displacement of the jugal 

 without apparent reference to function at all. The most extreme 

 reduction of the jugal is met with in the Murine forms, where the 

 whole arch is very slender and largel_y composed of the maxillary 

 zygomatic process which approximates closely to that of the 

 squamosal. In the Duplicidentata, on the other hand, the jugal 

 forms almost the entire arch extending behind but not actually 

 uniting with the glenoid. There is rarely any marked post- 

 orbifcal process in the zygomatic arch in Rodents. 



Among the S. American Edentates the zygomatic arch is 

 complete only in the Dasypodidae, where considerable differences 

 are exhibited even among members of the same genus. In 

 Dasypus sexciuctus, Chlami/dophorus, and in Priodontes gujas the 

 jugal curves downwards and articulates with the squamosal by a 

 horizontal fissure ; and in the first two genera there is a slight 

 indication of the downwardly directed process so characteristic of 

 the Sloths and Glyptodontida?. In Tatusia hybrida the jugal also 

 curves downwards, but ai'ticulates with the squamosal vertically. 

 In Tatusia novevicincta the arch is represented by a broad 

 straight bar mainly composed of the jugal element. In the 

 remaining recent families the jugal fails to meet the zygomatic 

 process of the squamosal, thougli in the Bradypodidaj it is a lai'ge 

 bone characterized by a ventral prolongation. lu the M^'rmeco- 

 phagidte the jugal is slender and often styliform, while the 

 zj'gomatic process of the squamosal is represented by a blunt 

 projection except in Mynnecophaga tamandua, where it is drawn 

 out into a tapering process that almost meets the jugal. There 



