SKULL MAMMALS 



213 



development of masticatory muscles the ramus of the lower jaw is low, the 

 coronoid process slight and the condyle rounded. 



Ungulata. — The groups of mammals usually included in an 'order' 

 Ungulata are now regarded as only distantly related to each other and the 

 group as polyphyletic, Artiodactyls and Perissodactyls, being remote from each 

 other, the latter group being more closely related to, among recent forms, the 

 Proboscidia, Hyracoidea and possibly Sirenia. For convenience the older 



'A:^,^v^^ 



Fig. 226. — Cranium of Zeuglodon (von Stromer in Jaekel, '11). /, frontal; >n. maxilla; 

 n. nasal; p, parietal; pm, premaxilla; s, squamosal; s, zygomatic. 



'order' is retained here, although there are few cranial features which are 

 common to all and at the same time distinctive of 'Ungulata.' 



The Artiodactyla, a group based on foot structure, have a great variety of 

 skulls. The cranium (largely pneumatic) has a very small brain part ; the occipi- 

 tal is nearly vertical or inclined backwards; orbit and temporal fossa may be 

 separated superficially by postorbital processes of frontal and zygomatic, or 

 these may not meet, only partially separating the two spaces. The supraoccipi- 



FlG. 



227. — Skull of Oreodon (Scott, '90). /, lacrimal; m, maxilla; n, nasal; o, 

 occipital; pg, postglenoid process; p, parietal; sq, squamosal; s, zygomatic. 



tal, broadly triangular in Ruminants, usually contains the interparietal, and the 

 parietals are fused in the median line, and in the Cavicornia the common bone 

 thus formed is very small and is forced to the occipital surface by the great 

 development of the f rentals; in other groups the parietal is far back on the roof 

 of the cranium. The nasals, usually elongate, are but shghtly broadened behind. 

 With the loss of upper incisors the premaxillae tend towards degeneration. In 

 the pigs they are supplemented by cartilage and prenasal bones in the snout. 



