SUBORDER A EMBRITHOPODA 247 



or serial; centrale often present. Extremities specialised in various ways, and in 

 aquatic forms the posterior pair is totally reduced. Skull elongated and low, with a 

 broad parietal region. Uterus bicornuate, placenta zonal, usually deciduous. 



On account of the high degree of specialisation which the skeletal elements 

 have undergone in the different suborders, a more precise diagnosis is 

 impossible. The suborders appear almost contemporaneously in Egypt, 

 being represented by forms with a low skull, a broad occiput, well-developed 

 sagittal crest and a narrow cranium, and usually the enlargement of a pair of 

 incisors, combined with a reduction of the remaining incisors and canines. 

 The molars are bunodont, brachyodont and practically quadritubercular. 

 Their cusps are combined in pairs to form ridges which become crescent- 

 shaped in the lower jaw. The premolars become molariform in early forms, 

 and the incisors, canines and premolars gradually pass into one another 

 when one pair of incisors is not enlarged. Also the development of the base 

 of the skull is very much alike in all of these forms. 



The extremities show very varied lines of development even in the oldest 

 Subungulates. The oldest Hyracoidea, apart from their bunodont dentition, 

 have all the external characteristics of recent forms ; on the other hand, the 

 genus Moeritherium approaches the primitive Proboscidea and Sirenia, not only 

 in dentition, but also in the structure of the humerus, scapula and pelvis, so 

 that the common ancestor of both can not be far removed. The Emhrithopoda 

 show, in spite of their very different dentition, so great a similarity in foot- 

 structure to the Proboscidea that a common ancestor, although rather remote, 

 is certain. 



That all four suborders must have arisen from a common bunodont form 

 long before the Upper Eocene, is shown among other characteristics by the 

 lack of the primitive feature of the entepicondylar foramen of the humerus. 

 Relationship to the Notoungidata must be absolutely excluded, for the denti- 

 tion of these South American forms originated directly from a secodont 

 ancestor, whereas the ancestor of the Subimgidata must have had a bunodont 

 dentition, from which we might easily infer a close relationship to the 

 Condylarthra. 



Suborder A. EMBRITHOPODA Andrews. 



Huge herbivores with lophodont molars. Fore limbs similar to those of the 

 Proboscidea. Hind limbs like those of the Amblypoda. Oligocene of Egypt. 



Family 1. Arsinoitheriidae Andrews. 



Huge ungulates with a pair of large bony horns on the nasals and two smaller 

 ones on the frontals. Short, high snout with nares divided anteriorly. Parietal 

 region inclined forward, with a large supraoccipital swelling. Orbits open 

 posteriorly. Dentition complete. Incisors, canines and premolars passing from 

 one form to the other and set very closely together. Premolars simpler than the 

 molars. Upper and lower molars made up of two y -shaped ridges, whose anterior 

 branch is shorter than the other. Humerus and femur long and massive. Tibia 

 and fibula short ; ulna much thicker than the radius. Astragalus flat with slightly 



