SOO Troxell — Biceratliermm and the Diceratheres. 



of the four upper premolars, the large upper incisor (see 

 fig. 1 F), and the distal end of the tibia. The premolars 

 {^g. 3) measure in length three fourths those of D. 

 armatum, the incisor less than two thirds. The species 

 differs so markedly from the type species of the genus 

 that it might justly be put in some other group: (1) the 

 crochet is almost lacking, possibly due to wear, the crista 

 being much more distinct; (2) the cingula are entirely 

 obsolete on the inner side of the deuterocone, but are 

 strong around the tetartocone; and (3) the size is much 

 smaller. 



In certain features this species is more progressive: 

 the deeper pits and sinuses, the cristse, and the grooves 

 marking off more distinctly the deuterocone in P^-*. It 

 has a close similarity to Menoceras coohi in its size, 

 broken cingula, and the general form of the cross lophs ; 

 but the differences, especially the foldings of enamel, are 

 greater and more fundamental. 



Peterson (1920) is justified in putting Diceratherium 

 nanum Marsh in a minor taxonomic position, because 

 the type is so incomplete ; there is, however, a difference 

 from D. annectens of one fifth in the size of the incisors, 

 the only parts duplicated. The worn teeth and broken 

 skull of the holotype of D. nanum are nevertheless of 

 value in showing the reduction of the incisors to the 

 formula, IJ, and in showing the long diastema between 

 the incisors and premolars, and the true D. armatum 

 type of horn cores. (See fig. 2.) 



Fi: 



Fig. 2, — A comparison of the horn rugosities in {A) Menoceras cooki 

 (Peterson), gen. nov., Cat. No. 10273, Y. P. M., where they are rounded 

 knobs; and {B) Diceratherium nanum Marsh, holotype. Cat. No. 10004, 

 Y. P. M., showing the broad nasals with elongated narrow ridges typical 

 of all true diceratheres. X l/^* 



