TRICERATOPS PRORSUS. 131 



narrow median bridge, as shown in a skull (No. 970, Am. Mus. Nat. Hist.) lately described 

 and figured by R. S. Lull (see fig. 26), who refers it to Triceratops serratus of Marsh. 



These elements are not preserved in the present specimen, and it is not possible to deter- 

 mine their nature. Posteriorly the narial orifice is confluent with the orbits, and the lachrymal 

 or infraorbital foramen opens directly into the narial orifice, doubtless having communicated 

 through it with the orbit. 



Seen from behind (PI. XXXIII) , the supraorbital horn cores appear close together and the 

 great hood-like frill is deeply arched, extending far beyond the occipital condyle and describing 

 nearly three-fourths of the circumference of a complete circle. For a distance of about 20 centi- 

 meters from the posterior border the inferior surface of the frill presents the same rugose and 

 grooved appearance as the upper surface, showing that this portion of the frill was covered below 

 also with horn or some other dense substance. Within this external area, which has an average 

 width of about 20 centimeters, the lower surface of the bone presents a smooth surface and 

 has the appearance of having been covered with the muscles and softer tissues. Far forward, 

 beneath the great frill, in the center, the occipital condyle is seen, its articular surface pre- 

 senting the appearance of an almost perfect hemisphere. It is borne on a short peduncle, 

 and just above it is the foramen magnum with a subtriangular opening. On either side of 

 the occipital condyle opposite the foramen magnum the exoccipital processes are given off. 

 These extend backward and outward and their distal extremities are expanded into broad, 

 thick blades, the extremities of which abut against low but strong buttresses running diag- 

 onally across the inner surfaces of the squamosals near their anterior ends. The inferior angle 

 of the exoccipital process protrudes a little below the inferior border of the squamosal in the 

 present specimen, as in the type of T. brevicornus. Beneath and a little in front of the exoc- 

 cipital processes the short, stout basioccipital processes are seen. These are exceptionally 

 short in the present species, and in front they are in contact with the basisphenoidal processes. 

 The latter are longer and more slender than the basioccipitals and articulate distally with 

 the inner of the two posterior branches of the pterygoids, while the much broader external 

 branch of the pterygoids laps over and articulates, by a peg and notch arrangement, with 

 the quadrates, which are continued downward and expanded transversely to form the condyle 

 for articulation with the shallow cup on the superior surface of the articular and surangular. 

 Distally the quadrates converge so that their inner inferior angles are separated only by a 

 distance of 185 millimeters from one another. 



As in the type of T. Jiorridus, the olfactory nerves leave the brain by two foramina in the 

 present specimen. 



The principal measurements of the skull and lower jaws are as follows: 



Measurements of skull and lower jaws of Triceratops prorsus. 



Mm. 



Length of skull from apex of nasal horn core to middle of parietal as restored 1, 383 



Greatest length of skull . 1 , 523 



Greatest width of frill . 944 



Expanse of antero-inferior borders of squamosals 556 



Expanse of epijugals 532 



Distance from orbit to apex of rostral _ . 712 



Distance from orbit to apex of nasal horn core 660 



Expanse of orbits 245 



Thickness of postfrontals back of orbit 155 



Distance from inferior border of orbit to lower extremity of jugal. 340 



Greatest diameter of orbit 128 



Least diameter of orbit 122 



Antero-posterior diameter of supraorbital horn core at base 195 



Transverse diameter of supraorbital horn core at base. 150 



Transverse diameter of supraorbital horn core 25 cm. above base 75 



Antero-posterior diameter of supraorbital horn core 25 cm. above base 105 



Circumference of supraorbital horn core 25 cm. above base 290 



a Bull. Am. Mus. Nat. Hist., vol. 19, pp. 685-695. 



