1 1 6 THE CERATOPSIA 



The common factors which may be used are: Size, always considering the indicated individual 

 age of the specimen. Thus, the Triceratops prorsus type skull is that of an old animal, as the 

 sutures are almost entirely obliterated, and yet it is the smallest known Triceratops skull, with an 

 over-all length of but 5 feet 1 inch. On the other hand, the Triceratops serratus type skull, while 

 quite young, is 5 feet 1 1 inches, and the Triceratops calicomis, not fully adult, 6 feet 1 0^4 inches. 

 This closure of the sutures does not necessarily coincide with sexual maturity, but with completion 

 of growth. 



Skull proportions is another factor ; thus, the muzzle may be either long or short, and the crest 

 either broad, in proportion to its length, or relatively narrow. 



The form of the rostrum has also been taken into account, as has that of the jugal, and jugal 

 notch, also the presence or absence of the pseudopineal fontanelle, the form and degree of inclina- 

 tion of the orbit, and the form of the infratemporal fossa. But the most characteristic elements in 

 all ceratopsians are the horns and the crest; and of these, the nasal horn seems to have the greatest 

 value as a specific criterion, for its form and size, if present, or even its absence, are all weighed. 

 The brow horns are perhaps less diagnostic, for a grouping of specimens according to brow-horn 

 resemblances does not coincide with a grouping based upon the nasal horns, and, moreover, the 

 former seem to show more individual variation, especially in curvature, than does the nasal; but in 

 no two skulls are either of these elements exactly alike. 



The vascular impressions, on the outer surface of the crest and facial bones, are an age criterion 

 rather than a specific one, for in older animals the horny integument became either heavier or more 

 closely fitting, possibly both. The extent of these markings on the inferior aspect of the crest 

 seems, however, to have been more important, for in some cases they are absent, again they are 

 limited to a well defined peripheral zone, or they may be more widely diffused. 



On the basis of these data, the following conclusions as to specific distinction or synonymy are 

 reached : 



Triceratops horridus Marsh 60 



Holotype: No. 1820 Y.P.M.; 01 greater portion of a skull, with parts of the lower jaws. 



Horizon: Lance formation. This specimen is number 31 out of 32, and therefore about the base of the sequence 



(seep. 115). 

 Locality: Niobrara County, Wyoming. 

 Collectors: J. B. Hatcher, C. A. Guernsey, and E. B. Wilson, 1889. 



The main characteristics of this species, which is the genoholotype, are: The skull is that of an 

 aged, and hence full-grown individual, having an estimated over-all length of 6 feet, 8 inches. 

 This is a large specimen, but not the largest of the species. Of the crest only the proximal part 

 is preserved, and but a small portion of the orbit; otherwise, most of the essential details can be 

 made out. The muzzle is fairly long, and the rostral very heavy, with deep vascular impressions. 

 The cutting edge is not so sharp as in some species and is downwardly curved toward the tip. 

 The nasal horn is broad at the base, short and blunt, with the dorsal contour in line with that of the 

 nasals, which is a prevailing feature of Triceratops, except in the elatus-calicornis phylum. The 

 anterior profile of the nasal horn slopes slightly toward the rear. The brow horns are exceedingly 

 stout and rugose, were probably fairly long, and slope forward, as in prorsus. Their base is 

 extremely heavy and is elliptical in cross-section. The orbit is not sufficiently outlined for charac- 

 terization in the type, but it seems to have been elliptical, with the long axis inclined at an angle of 

 15° or more from the perpendicular. The descending limb of the jugal is robust, with the longi- 

 tudinal axis nearly vertical, but sloping slightly backward, and with a bluntly rounded median keel 

 which nearly bisects the bone. There is no trace of the epijugal as a separate element. The jugal 

 notch is fairly deep, but the outline of the infratemporal opening is not preserved. The crest 

 proportions are not obtainable, but the preserved base is heavy, with deep vascular grooves on the 



80 Marsh, O. C, 1889, A, pp. 334-335 (Ceratofs horridus); 1889, B, p. 173 (Trieeratop horridus). 

 61 Hatcher, Marsh, Lull, 1907, PI. XXVI. 



