TRICERATOPS SERRATUS. 125 



Owing to the state of preparation of the type it is not possible to determine accurately 

 the characters of the quadrate, though it is hardly probable that it differs materially from 

 that element in other species of the genus. 



The postfrontals are much the most massive elements of the frontal region of the skull. 

 They give rise to the supraorbital horn cores and form the posterior half of the orbital border. 

 Anteriorly they articulate with the frontals and prefrontals above and the jugals below; infe- 

 riorly they are in contact with the jugals and squamosals, and posteriorly with the squamosals 

 and parietals. In the median line, just back of the horn cores, they are perforated by a large 

 foramen, 3 the pineal of Marsh, which in other skulls is known to communicate with certain 

 large cavities in the postfrontals situated back of the orbit, some of which, at least, are confluent 

 with the large cavities at the bases of the supraorbital horn cores. The floors of these large 

 cavities are formed in part, at least, by the supraoccipital, and their purpose seems to have been 

 to combine strength with lightness by that mechanical adaptation so frequently resorted to 

 in the animal kingdom, which finds its most perfect development among the Dinosauria 

 in the vertebrae of the more highly specialized Sauropoda. Anteriorly the postfrontals are 

 overlapped by the frontals and prefrontals, but posteriorly they overlap the parietals, and a 

 median projection of the latter element is inserted into a broad emargination of the prefrontals. 

 Only the bases of the supraorbital horn cores are preserved in the type, the upper half of both 

 being entirely wanting. The portions preserved show that these horns in the present specimen 

 were comparatively somewhat more slender than those of most other species of this group. 

 It is possible that this character may have been of sexual rather than specific importance and 

 that the type may represent the skull of a young female, since it would not seem improbable to 

 suppose that the horns would be less robust in the females than in the males. At the base of 

 each horn core on the posterior surface of the postfrontal just above the squamosal border there 

 is an elevated rugosity which is a continuation of that series of similar rugosities described 

 and figured by Marsh as being present on each squamosal. 



The squamosals are comparatively broad, short, and stout. The inferior border of each 

 describes the arc of a circle with a regularly sculptured peripheral border. The anterior extremity 

 presents an irregular border for contact with the postfrontal, jugal, quadrate and quadratojugal, 

 as shown in PI. XXVII. The surface throughout is rugose and grooved, presenting an intricate 

 system of shallow, ramifying channels, which doubtless lodged and gave protection to that 

 system of nutrient blood vessels which lay between the bone and a closely fitting horny covering 

 with which in life it was probably insheathed. 



The parietals, if ever separate, are now firmly coossified, demonstrating that these elements 

 became fused very early in the life of the animal. Taken together the parietals are very broad 

 behind and narrow in front, where on either side they present an elongated supratemporal 

 foramen which in other individuals is known to have communicated both with the temporal 

 fossa and with the cavities at the base of the horn cores. The median line of the parietals is 

 elevated antero-posteriorly and presents a series of four rugose prominences. The superior 

 surface of the parietals is convex transversely and concave antero-posteriorly. The external 

 surface of these bones, like that of the squamosals, is rugose, throughout and presents an intricate 

 system of shallow grooves for the lodgment and protection of blood vessels. The more impor- 

 tant of these are seen to emerge from the posterior borders of the supratemporal foramina as 

 larger and deeper canals, whence they branch and spread out over the surface of the bone, 

 becoming smaller and less marked with each successive ramification. As with the squamosals, 

 in life these channels doubtless lodged and gave protection to that system of blood vessels which 

 gave nourishment to the parietals and their insheathing substance, whether composed of horn 

 or other material. The sutures between the parietals and squamosals and the parietals and 

 postfrontals have remained open. 



The frill. — Taken together the squamosals and parietals form a broad frill, with the trans- 

 verse diameter nearly double the fore and aft, deeply concave below and broadly convex above, 



o See footnote on p. 24. 



S 



