EEDUCTION OF THK JUGAL IN MAMMALIA. 75 



The jugal bone in tlie Ungulates ai'ticulafces with the lacrymal, 

 extending beyond the anterior border of the orbit, as in the 

 Xenarthra, and constituting an appreciable part of the facial 

 region of the skull. The zygomatic arch is universally complete, 

 and at its posterior end is nearly always flattened. Among the 

 Tapiridye and Khinocerotidae the jugal is least reduced in the 

 Perissodactyle forms, where the orbit is not enclosed. In the 

 Equidaj the posterior extremity of tha jugal do3s not extend 

 vertically behind the orbital ring which is completed by the 

 junction of the frontal with the squamosal. In the Pecora, 

 •Tragulina, and Tylopoda the zygomatic arch is so similar as to 

 separate these three groups widely from the Suina. The orbit is 

 enclosed by the union of the jugal with the frontal, and the 

 former does not extend far behind it postero-ventrally. But in 

 the less specialized 8uina, where there is no separation of the 

 orbit from the temporal fossa, the jugal may extend back to the 

 glenoid cavity as in Stts and Dicotyles. Here the same tendency 

 for the displacement of the jugal l)y the zygomatic process of the 

 s(|uamosal manifests itself, as has been referred to already in 

 Pinnipedia. In Hippopotanuts alone does the enclosure of the 

 eye-socket occur — and not invariabl}'^ throughout the genus ; but 

 the structure of the orbital ring when complete difiers in this case 

 from that of the higher Artiodactyla in that the jugal and the 

 frontal meet one another on the anterior rim of the orbit internal 

 to the lacrymal bone. 



The jugal is not prolonged into the facial region l)eyond the 

 lacrymal in the Hyracoidea, the condition of the zygomatic arch 

 being similar to that which obtains in the Marsupials and some 

 Rodents. The jugal enters largely into the formation of the 

 glenoid cavity : in no ma.mmals do the maxilla and squamosal play 

 less part in the formation of the arch ; and if the modern 

 Hyracoidea provide any indication of the type of zygomatic arch 

 prevalent in the protungulate stock, although it may be admitted 

 that the extension of the jugal into the facial region is a secondary 

 modification, the considerations stated with legard to the reduc- 

 tion of the jugal both in the Perissodactyles and Artiodactyles are 

 nevertheless greatly reinforced. 



The progressive reduction of the jugal is clearly illustrated in 

 Andrews's work on the ancestry of Elephants. In the modern 

 forms the m.ixilla is extended backwards in a stout zygomatic 

 process excluding the union of the jugal with the lacrymal ; and 

 the squamosal approaches it upon the dorsal side of the arch. The 

 jugal, which is reduced to a nariow segment interposed between 

 them, sends back a slender limb postero-ventrally wdiich takes part 

 in the formation of the glenoid. The participation of the jugal in 

 the jaw articulation does not appear to be a secondary condition, 

 for, as Dr. Andrews* observes, in Jlceritherimn "the jugal is 

 large and f oims the-greater part of the zygomatic arch. Posteriorly 



* Andrews, C. VV. Tageblatt deb V. Int.'Zool. Cong-. IJerlin, No. 6. Phil. Trans, 

 vol. 196. 



