22 



THE CEEATOPSIA. 



than in those where they are more perfectly closed, regardless of the genus to which the species 

 may pertain. This is especially noticeable in Nos. 1821 and 1823, Yale Museum, the types, 

 respectively, of Triceratops (Sterrliolophus) flabellatus and Triceratops serratus, in both of which 

 most of the cranial sutures are still open indicating that the animal was in each instance com- 

 paratively young. The great diversity in size and form displayed by the squamosals in the 

 Ceratopsida? is well shown in PI. III. 



The parietals, though perhaps separated in extremely young individuals, early became 

 perfectly united, leaving no trace of a suture. They are imperfectly preserved in the type of 

 Triceratops flabellatus, but are quite complete in a number of other specimens in the Yale and 

 U. S. National Museum collections, and especially so in the type of T. serratus, No. 1823, Yale 

 Museum, as shown in fig. 16 and in PI. XXVIII. The following description of these elements 

 will be based largety on that skull. Superiorly they are convex laterally and somewhat concave 

 longitudinally, especially along the median line. Together they form about one-half the crest 

 or frill of the skull. Laterally and posteriorly they articulate with the squamosals throughout 

 about two-thirds of their length on the superior surface, when this articulation or contact is 

 interrupted on either side by the large supratemporal foramina or fossae. On the inferior side 

 or surface the parietals and squamosals are in contact throughout their entire length save at the 



point where the exoccipital processes 

 are interposed between them, as shown 

 in fig. 6. Anteriorly the parietals are 

 overlapped by the postfrontals and 

 underlapped by the supraoccipitals and 

 exoccipitals. They are much expanded 

 posteriorly but contract rapidly ante- 

 riorly. On their superior surface along 

 the median line or crest there is a series 

 of rugose prominences, usually three 

 in number. The posterior border sup- 

 ported seven low pointed epoccipitals, 

 one median and six lateral arranged 

 three on either side. Though there is 

 apparently some variation in the num- 

 ber of these ossicles it hardly seems 

 probable that such variations are of 

 specific importance or that the number was constant in different individuals for any given spe- 

 cies. Seven seems also to have been the number present in No. 1822, Yale Museum, the type 

 of T. prorsus, though the parietal is not entirely perfect in that specimen and it is therefore 

 impossible to determine the number with certainty. 



Pig. 16.— Superior view of skull of Triceratops serratus (type) , No. 1823, Yale 

 Museum, s, Squamosal; p, parietal; c, supratemporal fossa; e, epoccipital; 

 d, epijugal; x, postfrontal foramen; fp, postfrontal; pf, prefrontal; /, frontal; 

 j, jugal; h, supraorbital horn core; n, nasal; h', nasal horn core; pm, premax- 

 illary; r, rostral. One-twentieth natural size. After Marsh. 



THE QUADRATOJUGAL ARCH. 



The lower temporal or quadratojugal arch is formed by the jugal, quadratojugal, and 

 quadrate, and these bones are united by suture only even in comparatively old individuals, as 

 shown in fig. 10. 



The quadrate, fig. 17, A and B, is triangular in outline. When in position the longer 

 border or base of the triangle is almost parallel with the vertical axis of the skull, though 

 directed somewhat backward and upward. The inferior border is nearly straight and heavy 

 throughout its entire length, and might very appropriately be called the shaft, to distinguish 

 it from the extremely thin angular superior border. The anterior extremity of the quadrate, 

 although much narrower than the posterior, is heavier and presents an elongated and trans- 

 versely placed articular surface for articulation with the lower jaw. The posterior end of the 

 quadrate is wider but much thinner than the anterior and is overlapped by the squamosal and 



