BIG BADLANDS OF SOUTH DAKOTA. 479 



undifferentiated from the rest of this structure, except for a small 

 swelling which perhaps corresponds to the entoconid. Unfortu- 

 nately, these observations have to be based on a single tooth, for 

 the corresponding molar on the right side has the crown badly shat- 

 tered. There is less marked contrast in length and width between 

 mf and m2 than in the second lower jaw to be described, No. 12546, 

 which, unfortunately, has lost the mental processes, but agrees closely 

 with the dimensions given by Mr. Troxell for the paratype of 

 Marsh's Arch<zotherium crassum, as may be gathered from the table 

 of measurements given below. In this specimen (Fig. 10) pi is 



Fig. 10. Arthccotherium crassum? (Marsh). No. 12546. Crown view of the 

 lower teeth of the left side, one third the natural size. 



definitely double-rooted, 10 mm. back of the canine and the same 



distance from p2, which is 10-12 mm. from p3, and the latter 4 mm. 



from p4, which carries a broad mammillated heel, tapering in width 



posteriorly. The anterior molar seems proportionately smaller in 



comparison with the tooth back of it than in the other specimen, 

 there is an external cingulum about the hypoconid in m2, and m3 



has the normal heel development with two major cusps and a mam- 

 millated, cingulum-like hypoconulid. There is also close agreement 

 in size with Leidy's fragmentary type of A. robustniUy^ where the 

 hypoconulid in m3 is a little stronger than in the Princeton specimen. 

 The table of measurements below shows a close approximation to 

 crassum and a rather wide departure irom.inortoni, and, in view of 

 the extremely fragmentary character of the type of robustum, later 

 referred by its author to mortoni, I am inclined to identify our 

 specimen, provisionally, as A. crassum. 



Still another form, perhaps to be referred to Leidy's A. ingens, 

 is represented by a fragment of mandible in the Princeton collection. 

 No. 10875, from the lower Oreodon beds in Corral Draw, South 

 Dakota, where the single anterior mental process preserved is of the 

 ingens type. 



^°" Ancient Fauna of Nebraska," PI. X., Figs. 12, 13. 



