MEMOTRS 
OF THE 
Coa wor MUSEUM. 
VOL Ii: NO. 2. 
THE OSTEOLOGY OF THE CHALICOTHEROIDEA WITH SPECIAL 
REFERENCE TO A MOUNTED SKELETON OF MOROPUS 
ELATUS MARSH, NOW INSTALLED IN THE 
CARNEGIE MUSEUM. 
By W. J. HoLtuanp AND O. A. PETERSON. 
(Plates XLVITI-LXXVII.) 
INTRODUCTORY. 
In the spring of the year 1904 a field party from the Carnegie Museum under 
the control of Mr. O. A. Peterson was engaged in collecting in the Tertiary beds 
on Monroe and Van Tassel Creeks and the Upper Niobrara River, locally known 
as ““The Running Water,” in western Nebraska. At the end of July Mr. Peterson, 
who several times had been courteously invited by Mr. James H. Cook, the pro- 
prietor of the Agate Spring Stock Farm, to visit him, accepted Mr. Cook’s hos- 
pitality. The ranch of Mr. Cook is located on the Niobrara River, in Sioux 
County, about twenty-five miles southeast of Harrison, the county-seat. A few 
days after the arrival of Mr. Peterson he was conducted by Mr. Harold Cook, 
the oldest son of his-host, to the locality just beyond the eastern limits of Mr. 
Cook’s property, designated as “Quarry A” in the map (Fig. 1) accompanying 
this memoir. Here young Mr. Cook had noted that numerous fragments of fossil 
bones were lying upon the surface. Mr. Peterson immediately took steps to 
begin the work of systematic excavation at this point. A few days afterward, 
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