148 THE CEEATOPSIA. 



margin of each of these two inferior processes of the pterygoids are produced into two broad 

 laminae, which run obliquely upward across the inner surface of the bone, the free borders curving 

 toward one another and partially arching over a deep canal, broad below but constricted above, 

 which may have functioned as the eustachian canal. 



Above the upper limits of these two laminse the pterygoids are much constricted and the 

 superior process presents a smooth, flattened, external articular surface for contact with the 

 palatines, while on their internal sides they articulate with one another through short grooved 

 articular surfaces, which are partially interrupted near their superior borders by a single 

 large median foramen. The sutural surface for contact with the palatine is projected above 

 that which opposes the pterygoid, and there is a small foramen on the free border between 

 these two surfaces, as shown in fig. 23. The anterior edge of the pterygoid extends forward 

 and forms an elongate plate, crescentic in outline. This articulates with the palatines above 

 and below with the maxillary, as shown in fig. 21. Just above the maxillary a large foramen 

 passes between the pterygoid and palatine. Anteriorly this foramen is bounded by the poste- 

 rior border of the palatine and posteriorly by the anterior border of the pterygoid. At the 

 extreme top the pterygoids articulate externally with the palatines, and to a slight extent 

 also with the inferior lateral projections from the vomers, while medially they are in contact 

 with one another, save for the interruption due to the median foramen already mentioned, 

 which doubtless served, to transmit the sensory nerves to the palate. 



The basisphenoid. — The basisphenoid is firmly coossified with the alisphenoids. The 

 basisphenoidal processes are produced somewhat beneath the basioccipital processes and pre- 

 sent in front at their extremities rugose surfaces for contact with the pterygoids. Anteriorly 

 and superiorly the basisphenoid is compressed and forms a stout median interorbital septum. 

 The external opening of the middle eustachian canal is situated between and at the base of 

 the basisphenoidal processes, as shown in fig. 23. It is entirely within the basisphenoid instead 

 of being situated between that bone and the basioccipital, as in the crocodile. Two large 

 foramina, situated one on either side of the skull at the bases of the basisphenoidal processes, 

 pierce the basisphenoid and enter the brain case near the base of the olfactory lobe through a 

 deep fossa, which doubtless lodged the pituitary body. Anteriorly the basisphenoidal proc- 

 esses are received into deep pockets on the posterior surfaces of the thin but widely expanded 

 posterior wings of the pterygoids. 



The alisphenoids. — The alisphenoids, including also the parasphenoids, with which they 

 are so completely fused, even in young individuals, as to render the latter elements indistin- 

 guishable, are extremely irregular in form. They are firmly coossified with one another and 

 with the exoccipitals and the basisphenoid. Together with the latter element the} 7 usually 

 form the entire anterior portion of the brain case, save only the extreme anterior portion of 

 the superior border, which in some instances is formed by the anterior projection of the united 

 postfrontals. Supero-posteriorly the alisphenoids articulate with the supraoccipital, and 

 supero-anteriorly with the postfrontals. Just beneath the lateral union of the supraoccipital 

 and postfrontals the alisphenoids are developed into a strong lamina or buttress, which gives 

 greater support to this region. Anteriorly the coossified alisphenoids and basisphenoid are 

 embraced by the vomers and the posterior projections of the palatines, as is well shown in fig. 

 24, from the type of Triceratops horridus, No. 1820 of the Yale Museum collections, though 

 not so apparent in the type of Sterrholophus fiabellatus, in which these parts are less perfectly 

 preserved. In the type of Sterrholophus fiabellatus the olfactory nerves, as shown in fig. 9, 

 leave the brain case by a single large median foramen inclosed entirely by the alisphenoids, 

 while in the type of Triceratops horridus this foramen is subdivided by a strong but short median 

 partition of bone, as shown in fig. 27. This difference has been shown to be due to age. (See 

 p. 120.) 



Note. — No measurements of Sterrholophus fiabellatus were left by Hatcher, probably because of the disarticulated con- 

 dition of the skull— R. S. L. 



