OF THE WHITE EIVER BEDS OF MONTANA. 



269 



the outside in the truncation of the apex of the tooth. A protoloph begins at the apex 

 of the deuterocone, and, becoming narrower and lower, extends to the outer anterior 

 buttress. The tetartocone is small but well defined. A cingulum surrounds the inner 

 part of the tooth. 



The molars are broader than long. They bear very low crescents. The antero- 

 external buttresses are large, especially on M». There are inner cingula on M 2 and 



M ;i and faint traces on M 1 . , 



The zygomatic arch is quite broad under the orbit ; its lower angle being opposite 

 the posterior part of M 2 . The anterior inferior root expands outward abruptly. One 

 root of M ;i projects through the maxillary into the orbital plane. Evidently the 

 posterior of the orbit was in the same plane as the posterior of the last molar. 



Measurements. 



Length of last two premolars and molars. 



Length of molar series 



Length ofP* 



"Width of PA 



Height of crown of P- 1 



Length of P± 



Width of PA 



M. 



.0400 

 .0250 



.0075 



.0062 



.0047 



.007 



.0084 



Height of PA 



Length of M- 1 - 



Width of Mi 



Length of M^ 



Width of MA 



Height of M* and B£*» each 



Length of SP 



Width of M^ 



Greatest width of zygomatic arch under orbit. 



.0042 



.0085 



.009 



.0088 



.0105 



.008 



.0103 



.012 



.013 



Arreiotherium acridens, gen. et sp. nov. (Plate IX, Figs. 1-3.) 



Type No. GO. 

 Of this unique animal I have most of the superior dentition and parts of the skull 

 and skeleton. It was found in a fine sandy clay near Blacktail Deer creek, twenty- 

 live miles south and east of Dillon, in the southwestern part of the State. Part of the 

 posterior portion of a skull of a rhinoceros— probably Camopus— about the size of G. 

 tridactylum, was found in a sandy layer near, and a considerable distance above the 

 type of Steneofiber hesperus was obtained. The bones are much broken and part of 

 them had been washed out and mixed in confusion in the clay. I saved all the frag- 

 ments I could find, taking some of the material to the creek and "panning it out." 



A. P. S. — VOL. XX. II. 



