416 MEMOIRS OF THE CARNEGIE MUSEUM. 



type specimen. This is due to the studies of Professor Marsh and later students. 

 Therefore it is only necessary to here state that the ectoloph is perhaps thinner, 

 the excavations or valleys of the crowns larger, and the cross-crests simpler, than 

 in D. annedens or later forms from the Nebraska Miocene. P^ is also observed 

 to be proportionallj^ large, and the cingulum seems to be well developed, especially 

 on the internal faces of the teeth. 



A case of reversion, or at least a non-uniformity of tooth-structure worthy of 

 note, is seen in the second specimen of D. armatum in the Yale Museum collection 

 (No. 10,055). The metacone on P- of the right side of this specimen displays a 

 curious primitive roundness, though connected with the ectoloph by the usual 

 thin cross-crest, while the corresponding tooth of the opposite side has this postero- 

 internal tubercle of the usual type seen in the Diceratheres. There is otherwise 

 little or no differences in the dentition from that in the type, except that No. 10,055 

 represents a younger animal. The crista, crochet, etc. are very little or not at 

 all indicated, while the cingula are prominent, especially internally. 



With the skull No. 10,055 of the Yale Museum collection, referred to D. arma- 

 tum, there are associated fragments of lower jaws, which undoubtedly belong 

 with the skull, inasmuch as the third molars, both upper and lower, arc not yet 

 entirely developed. These fragments of the lower jaw indicate that the hori- 

 zontal ramus was very heavy, but rather shallow, the symphysis strong, the mental 

 foramen of large size, and located directly below Pi. The roots of the lateral 

 incisors indicate that the crown was large and most likely of the usual type met 

 with in the family. The symphysis is broken off too far back to show any indica- 

 tion of the median incisors. Pi has a rather small antero-posterior diameter, but 

 the crown is quite high ; the tooth is broken externally and the grinding and internal 

 faces are buried in the matrix. This is also true of P2. The external face of the 

 latter tooth shows a very heavy cingulum, which extends around the entire pos- 

 terior face, but has a less upward oblique trend than is seen in the later forms 

 from Nebraska. The crowns of the cheek-teeth are little worn, indicating the 

 juvenile stage of the specimen. Mo has also a cingulum on the external face which 

 is, however, less developed than on P2; this is especially true of the posterior lobe 

 of M2. 



The fourth metacarpal associated with the type of D. armatum is rather long 

 and broad, having a comparatively small antero-posterior diameter. The bone is 

 somewhat crushed, but the proximal end is not distorted and indicates that the 

 bone was not very thick fore-and-aft. The distal trochlea extends well up upon 

 the anterior face of the metacarpal. Judging from the unciform, which is present, 

 the carpus was fairly high. 



