U, S. GEOLOGICAL SURVEY 



MONOGRAPH 55 PLATE CLXI 



SKULLS OF BRONTOTHERIUM LEIDYI 



One-fourth natural size. (See p. 559.) A, Skull R, type (Nat. Mus. 4249), Big Badlands, S. Dak., Chadron formitiDn, level middle A. Figure prepared 

 under the diredlion of Professor Marsh. This olde^ and mo^ primitive member of the Brontotherium phylum foreshadows the later bronto- 

 theres in the flattened oval sedlion of the horns, in the tapering decurved nasals, and especially in the progressive chara(5ters of the premolir 

 dentition. (See PI. CLXIII.) It is larger than the contemporary primitive members of the Brontops and Menodus phyla. B, Referred skull 

 (Carnegie Mus. 93), Hat Creek, Nebr., Chadron formation, level lower A ("15 or 20 feet from bottom of lower beds" — ^J. B. Hatcher). Figure 

 prepared under the diretflion of the author. This well-preserved skull has the horns a little longer than in the type. Male sex is indicated by the 

 large size of the c 



