PETERSON: THE AMERICAN DICERATHERES. 435 



142 mm. Mandibular symphysis very short. Locahty Colorado. Amer. Mus., 

 Cope Collection, No. 6325." 



This type specimen is accompanied by an upper maxillary with teeth and 

 other skeletal material, but, as there seems to be some doubt as to their association, 

 they will not here be considered. 



A second specimen in the American Museum Collection from the upper 

 Oligocene (Protoceras Beds) illustrated on page 139 of Osborn's work is especially 

 interesting. This lower jaw (No. 1110) is doubtfully referred to Coenopus {Acera- 

 therium) mitis Cope, and exhibits just such characters as one might expect to find 

 in an ancestral form of D. cooki: the short symphysis; the short diastema between 

 incisor and cheek-teeth; the curving of the lower border of the ramus; and the angle 

 everted as in D. cooki. The ramus itself is, however, deeper, the vertical ramus 

 having a gTeater antero-posterior diameter, and the coronoid process a more nearly 

 vertical position than in D. cooki, as indicated by the illustration. The measure- 

 ments (p. 140) do not appear to agree completely with the illustrations. 



This lower jaw undoubtedly represents a distinct species, judging from the 

 great vertical depth of the sediments lying between the Titanotherium beds in 

 which Coenopus mitis (Cope) was found, and the Protoceras sand-stones, together 

 with the general change in the fauna of these two geological horizons. This 

 lower jaw from the Protoceras beds may here be provisionally regarded as the type 

 of Coenopus dakotensis sp. nov. and also provisionally placed in the line more or 

 -or less directly leading to D. cooki found in the lower Harrison beds of Nebraska 

 as indicated in the table found in the introduction to this paper. 



Vertebral Column. 



The vertebral formula is provisionally given as follows: Cervicals, 7; Dorsals, 

 19 (?); Lumbars, 5; Sac7'als, 4-5; Caudals, 26. 



Cervical Vertebrce (Figs. 17-21). In a fully adult animal the width of the 

 atlas is almost double that of the length. The anterior projection of the trans- 

 verse process extends well forward. The neural canal is of moderately large size, 

 while the arterial canal on the ventral face of the transverse process is generally lack- 

 ing. The median area of the neural arch varies in different individuals in robustness 

 and rugosity. The transverse process and the median tubercle of the lower pos- 

 terior face of the body also vary in robustness and size. 



The odontoid process and body of the axis are heavy; the neural spine is 

 generally heavy and overhanging, while the transverse process projects rather 

 strongly backward. The latter process is subject to much variation in size, as is 



