36 Field Museum of Natural History — Geology, Vol. IV. 



would designate D. heterodon as a primitive form ; the position assigned 

 to it geologically indicates that it is the most recent yet described. It 

 is apparently one of those rare forms, immigrants or adaptations under 

 stress, which characterize the transitional stage. 



D. fluminalis displays a high degree of specialization in the post- 

 narial characters. The nares are bridged over so as to obliterate 

 almost all evidence of their primary position. In this process the 

 opening has receded to a point back of the hamular processes. 

 This recession, noted in less degree in other species, is evidence of a 

 secondary adaptation to aquatic habits of feeding. Other characters 

 of the skull in this species would not indicate that this animal was 

 aquatic in its general habits. Like many other terrestrial mammals 

 it probably fed upon submerged plants. The slenderness and delicate 

 modeling of the skull would suggest an animal lighter of limb and 

 more active than other species of this genus. In the development of 

 horns the type of this species is more advanced than the type specimen 

 of D. cornutus. Its narrower sagittal area, its strongly recurved canines 

 and much smaller molars readily distinguish it from that species. 



Rhadinorhinus gen. nov. 



Type species: R. abbotti. 



Range: Metarhinus Beds, Middle Eocene. 



Generic characters: Titanotheres with slender skulls, nasals deeply 

 recessed laterally and tapering, molars long-crowned, P 2 , 3 , 4 , sub- 

 rectangular, a wide median area between the incisors, no infra-orbital 

 process. The name "Rhadinorhinus" alludes to the tapering nasals 

 which characterize this genus. 



R. abbotti, sp. nov. 



Type specimen, No. 12 179 Field Museum. (PL XI, Figs. 2-3). 

 Type Horizon: Upper Metarhinus Beds. 



Specific characters: Length of skull 435 mm., molar-premolar series 



168 mm., nasals shorter than premaxillaries, thickened at suture 



and tapering toward a terminal rugosity. Arches slender, 



posterior nares open opposite middle of M 2 . Sagittal crest long 



and narrow. . Hypocone of M 3 vestigial, diastema short. 



This interesting specimen was discovered by Mr. J. B. Abbott, in 



whose honor it is named. It indicates a form closely related to both 



Mesatirhinus and Metarhinus, but probably differing from them in 



habits. The skull is slighter in construction, the arches more slender 



and the teeth longer in the crowns throughout. The frontal region is 



