GENERIC AND SPECIFIC SUMMARY. 169 



The nasal horn core is long and is directed forward, the anterior border extending forward 

 and upward at an angle of 30° instead of being perpendicular to the long axis of the skull as in 

 T. Jiorridus and T. hrevicomus. The horn much exceeds that of each of the contrasted species 

 in length, the tip of the horn in the type specimen being just over that of the rostral bone 

 (PI. XXXIV). 



The supraorbital horn cores are slender, and are directed upward, forward, and outward 

 at an angle of 45° for about half their length, then curve gently inward. Here the contrast with 

 the allied species is evident in the stoutness of the horns as compared with their slenderness in 

 T. prorsus. The horns of T. prorsus are much longer proportionately than those of T. hrevicomus. 



The orbits in the present species are nearly circular as contrasted with the elliptical orbits 

 of hrevicomus. The form of those of T. Jiorridus can not be ascertained, as but a quarter of the 

 margin is preserved, but they would seem to agree more nearly with those of T. prorsus. 



The lachrymal foramen is entirely within the maxillary bone, as in T. (Sterrholophus) 

 flabellatus, in contrast to its position between the maxillary and nasal as in T. serratus, T. hrevi- 

 comus being in a sense transitional between the two types, while the condition which obtained 

 in T. horridus can not be determined, as this part of the specimen is lacking. 



The frill is deeply arched transversely, ranging through an arc of 27°, with seven lateral 

 and one median epoccipitals, making fifteen in all. The quadratojugal notch is deeper than in 

 any other known species, and the postfrontal (pineal) fontanelle is entirely closed. Posteriorly 

 the frill border was free, for vascular impressions occur on its inferior face for a distance inward 

 of 20 cm. from the margin. This feature, together with the closure of the postfrontal fontanelle, 

 may be a characteristic of old age, but in some old skulls of other species the fontanelle seems 

 to be persistently open. 



3. Triceratops hrevicomus Hatcher (pp. 141-142, Pis. XL-XLII) presents another instance 

 of an aged individual, the type being No. .1834 of Yale Museum. 



The rostral bone is proportionately very heavy, with a deeply excavated inferior surface. 

 The nasal horn core is short and very stout, the antero-posterior diameter much longer than the 

 transverse, and the anterior border vertical, as in T. Jiorridus. 



The supraorbital horn cores are short and stout and, in contrast to the much longer cores 

 of Jiorridus and prorsus, more nearly circular in section than in any other species. 



The orbit is an irregular ellipse, and the lachrymal foramen lies between the nasal and 

 maxillary bones, but is partially embraced by an ascending process of the latter. 



The infratemporal arcade is formed from the quadrate, with overlapping processes from 

 the jugal and squamosal, as in T. serratus. The frill is elevated somewhat sharply toward the 

 posterior margin, as in T. prorsus, and bears nineteen epoccipitals, six pairs of which are borne 

 on the squamosals, as contrasted with fifteen in the last-named species. It is doubtful whether 

 or no this will prove a specific rather than an individual distinction (Hatcher). 



4. Triceratops serratus Marsh (pp. 122-127, figs. 16, 26, 32, 34, 42, Pis. XXVII-XXIX) is 

 founded upon the skull of an immature individual (No. 1823, Yale Museum), but one in a remark- 

 able state of preservation. 



The rostral bone is rather small, lighter and less rugose than in other species; this, however, 

 may be either a juvenile or possibly a sexual character. 



The nasal horn core is wanting in the type, having been lost at the suture between it and 

 the nasals. It must, however, have been considerably compressed transversely. 



The supraorbital horn cores are slender and much more erect than in most species, some- 

 what elliptical in section at the base and more nearly circular in their mid-length. 



The orbit is large and irregularly elliptical in outline, its long axis running obliquely down- 

 ward and forward. The position of the orbit is in advance of and superior to that of T. (Sterrho- 

 lophus) flahellatus. 



The lachrymal foramen lies between the nasal and maxillary, as in the last-mentioned 

 species. 



