PETERSON: THE AMERICAN DICERATHERES. 421 



due to the longer muzzle of the form from the John Day. The zygomatic arch of 

 the latter is somewhat lighter, especially at the posterior angle, which forms a 

 less direct right angle with the side of the skull and is not nearly so rugose. The 

 region back of the pterygoid processes is decidedly longer in the John Day species 

 than in the eastern species. Thus there is a wide separation between the post- 

 glenoid and paroccipital processes in the earlier species, while in the later form 

 {D. cooki) these processes are closely united. Even the occipital condyles of the 

 John Day specimen are less sessile. The anterior margin of the posterior nares 

 is opposite the interval between M' and M^ as in the smaller Nebraskan form, 

 D. cooki, and the size of the nares is of the same proportion. 



The premolars of the specimen in the American Museum are even more worn 

 than in the type, which is brought out in the illustration, PL LXIII, Fig. 6. They 

 are, however, not too far gone for identification and comparison, and they are 

 seen to agree with the type in New Haven. M^ has a decided antecrochet-like 

 swelling of the anterior lobe, which is as great as, or perhaps greater than, in any 

 of the Nebraskan specimens which I have seen. The crochet is of quite large 

 size and totally separated from the ectoloph, so that at no stage of wear will this 

 process apparently ever become united with the ectoloph as in D. cooki. M- has 

 the crochet generally less developed than in D. cooki. M' has on the right side a 

 curious basal cusp on the posterior margin of the exit of the median valley which is 

 very similar to the same tooth in a specimen of D. cooki at the Carnegie Museum, 

 though less deeply separated from the main body of the tooth. (See Fig. 4, p. 409.) 

 On M^ of the left side there is also a minute tubercle situated in a position similar to 

 the one described above. With the exception of the relative size of the median 

 incisor and the first premolars D. annectens from the John Day and D. cooki from 

 the Nebraska Miocene differ less in the detailed structure of the dentition than 

 was anticipated. 



On page 422 are tabulated measurements of the specimens above described. 



8. Diceratherium gregorii" sp. nov. (Plate LIX and text-figure 12.) 



Type. — Skull,? female. American Museum, No. 12,933. 



Horizon. — Miocene, Lower Rosebud beds. 



Locality. — Near Rosebud Indian Agency, South Dakota. 



Specific Characters: Occiput low, but overhanging, as in the John Day form 

 {D. annectens) Sagittal crest low, but well defined. Postorbital ridges converging 

 very gradually, as in the John Day form, but the brain-case proportionally larger in 

 size. Greater robustness of the inion, shorter basicranium and premaxillaries, when 



" In honor of Dr. W. K. Gregory, of the American Museum of Natural History, who found the type- 



