242 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



Observations on the measurements oj AUops wal- 

 cotti. — The type and only known specimen of this 

 species exhibits the following comparison in measure- 

 ments with skulls of B. hrachycephalus and Menodus 

 Jieloceras, which show that the type of AUops walcotti 

 has relatively large premolars and small molars. 



Measurements of AUops walcotti, Menodus heloceras, and Brontops 

 brachycephalus, in millimeters 



Etymology. — -"The species is named in honor of 

 the Secretary of the Smithsonian Institution, Charles 

 D. Walcott." (Osborn.) 



Present determination. — The skull is 'crushed later- 

 ally but probably had a low zygomatic index — that is, 

 it was mesaticephalic. While its reference to AUops 

 requires confirmation, its nearer affinities appear to be 

 with this genus rather than with Brontops or Menodus. 

 The external cingula of the premolars are not as 

 sharply defined as in other primitive members of the 

 menodontine group. 



Megacerops riggsi Osborn, 1916 



See page 550 



Original reference. — Am. Mus. Nat. Hist. Bull., vol. 

 35, p. 723, fig. 2, 1916 (Osborn, 1916.433). 



Type locality and geologic horizon. — Northeastern 

 Colorado, Horsetail Creek; Chadron formation (Titano- 

 therium zone), upper (?) levels. 



Type. — A nearly complete lower jaw in the American 

 Museum (No. 6364). E. D. Cope, collector. (See 

 fig. 208.) 



Specific characters (Osborn). — Of small size, smaller 

 than any known individual of Megacerops or Bronto- 

 therium. Very massive jaw with a small coronoid 



FiGUEE 208. — Type (holotype) jaw of Megacerops riggsi 

 Am. Mus. 6364. After Osborn, 1916. One-sixth natural size. 



process and a very short symphysis. Premolar series 

 greatly abbreviated (85 mm.). Premolars and molars 

 with reduced external cingula. 



Measurements of type 



Millimeters 



Symphysis to condyle (estimated) 465 



Premolar-molar series (pi-ms) 282 



Premolar series (pi-pi) 85 



Molar series (mi-ma) 194 



Etymology. — Named "in honor of Mr. E. S. Riggs, 

 of the Field Museum of Natural History, in recogni- 

 tion of his discoveries of Eocene titanotheres." (Os- 

 born.) 



Present determination. — The type of this species 

 is a jaw in the Cope collection (Am. Mus. 4636), 

 which was wrongly referred by Cope to his species 

 " Symhorodon" acer. It represents a highly specialized 

 and small form of Megacerops. 



Note. — For descriptions of upper Eocene and lower Oligo- 

 cene titanotheres from MongoUa described by Osborn in 1923 

 see appendix; also the final opinion regarding the titanotheres 

 of eastern Europe, page 941. 



