TRICERATOPS PRORSl'S 



117 



outer aspect. Those oil the under side cannot now be seen. As preserved, the dorsal outline is an 

 even curve with no prominences on the midline, as in setratUS. These may have been present, 

 however, farther back, but judging from referred specimens, I imagine not. 



To this species, 1 would refer the huge skull No. 1828 Y.P.M., as far as can be ascertained in 

 its present condition. This skull carries the manuscript name of "iiigens," in Professor Marsh's 

 handwriting, but, while that name has been published by Lull, it was without description and was 

 therefore, disallowed by Hay. It is the largest of all Triceratops skulls, measuring over 8 feet in 

 length, an aged individual, with deep vascular impressions on the outer surface. But it is next 

 to the uppermost of the Niobrara County series, showing the conservativeness of this species. 



To twin.! us 1 would also refer a skull (PL XIII, B) in the South Dakota School of Mines, 

 No. P271, collected in the Short Pine Hills, Harding County, South Dakota (see map, Fig. 2, 

 No. 1 1 ). This skull, which has an over-all measurement of 5 feet 6]/ 2 inches, and is that of a not 

 fully adult individual, judging from the condition of the sutures, agrees with the type in most 

 respects, especially in the character of the horns, but not in the form of the jugal, which however 

 is partly restored and may not be of great significance. 



Another large skull, No. 12,003 (PI. XIII, A), in the Field Museum of Natural History, 

 from the Lance formation of Chalk Buttes, Montana (see map, Fig. 2, No. 10), is also probably 

 referable to this species. It is 6 feet 9 inches in length. 



A large skull in the American Museum, No. 5028, was collected by Brown in 1908 from the 

 Hell Creek beds of Dawson County, Montana. This is the skull of an aged animal, and measures 

 6 feet 1 0^4 inches in over-all length, hence a little greater than the estimated length of the type, 

 though the actual measurements of the facial region are materially larger than in the type. The main 

 distinctions of this specimen, as compared with the type, lie in the larger nasal horn, which is more 

 like that of prorsus in having the tip extend forward to a point nearly over the anterior edge of 

 the rostral, and in the shorter and more erect brow horns. The descending limb of the jugal 

 differs also from that of the type in its narrower proportions. The skull differs from the type of 

 prorsus mainly in its much greater size and somewhat shorter horns, and in the apparent lack of 

 the defined peripheral zone of vascular impressions on the under side of the crest. From brevi- 

 cormts, this skull differs in greater size, longer nasal horn, and also in the lack of vascular zone. 

 The animal quite evidently belongs to the horridus-prorsus-brevkornus phylum, its closest affinity 

 being with horridus. It is unique in the disproportionately large facial region, as compared with 

 the relatively abbreviated crest, which simply emphasizes once more the infinite variation of the 

 ceratopsians. 



Triceratops horridus, although conservative, seems to have been a widespread species both in 

 space and in time. I would group horridus near prorsus and brevicornus, as opposed to the elatus- 

 calicornis phylum, although not necessarily in the same ancestral line. 



Triceratops prorsus Marsh'- 



Hol<>tvpc:'' ; No. 1S22 Y.P.M.; nearly perfect skull with lower jaw, and six anterior cervicals. 

 Horizon: Lance formation, No. 19 in the Niobrara County sequence. 

 Locality: Niobrara County, Wyoming. 

 Collector: J. B. Hatcher, 1889. 



The type skull is that of an aged individual, but one of small size; in fact, its over-all length of 

 but 5 feet 1 inch, makes it the smallest Triceratops species and specimen known. 



The nasal profile is nearly straight to a point about over the middle of the narial opening, 

 whence it curves gently upward and is continuous with that of the nasal horn. The profile of the 

 crest begins at a point below that of the nasals, so that the two are not in line when viewed laterally. 

 Posteriorly, the crest rises to a point about level with the tip of the nasal horn. The orbit is nearly 



62 Marsh, O. C, 1890, p. 82. 



83 Hatcher, Marsh, Lull, 1907, Pis. XXXII-XXXVI. 



