THE SKULL. 



17 



Fig. 7. — Occipital condyle of type of 

 Torosaurus gladius, No. 1831, Yale 

 Museum. bo, Basioccipital; exo, 

 exoccipital. One-half natural size. 



were early obliterated. They are not visible in any of the skulls known to me. The foramen 



lacerum posterius is large, as shown in fig. 8, and is situated at the base of and just anterior to 



the processes of the basioccipital and exoccipital. It is probable that the basioccipital and 



exoccipital and the basisphenoid and alisphenoid all took part in inclosing this foramen. 

 The suture between the supraoccipital and exoccipital remains 



open until late in life, so that the form and character of the supra- 

 occipital are easily determined. It is proportionally small and 



composed of two rather thin wing-shaped portions articulating 



inferiorly on either side with the exoccipitals and alisphenoids. 



Each of these is produced backward, so as to inclose and overhang 



the deep cavity already mentioned in describing the exoccipitals, 



and they unite medially to form the strong superior portion of that 



keel which divides this cavity vertically into two lateral cavities. 



Superiorly the supraoccipital sends upward two pillars, one on either 



side of the median line, to give support to the parietals at their 



union with the postfrontals, thus contributing to the firm support 



of the supraorbital horns. In this manner the supraoccipital 



unites with the widely expanded processes pf the exoccipitals in 



giving support from beneath to the enormous frill-like occipital 



crest formed by the squamosals and parietals. 



The condyle is pedunculate. The peduncle, or neck, is rather short, circular in cross 



section, and is usually somewhat constricted. The basi- 

 occipital and the two exoccipitals enter about equally into 

 its construction, though the former is excluded from the 

 foramen magnum. The articular surface of the condyle 

 has the form of a nearly perfect hemisphere, and indicates 

 considerable freedom of motion, notwithstanding the 

 depth of the cup in the atlas and the enormous posterior 

 projection of the frill. The condyle of the type of Tricera- 

 tops prorsus (No. 1821, Yale Museum) is 91 millimeters 

 in vertical diameter and 102 millimeters in transverse 

 diameter. 



The foramen magnum is small and somewhat ellipti- 

 cal in outline, the vertical axis being the longer. It has 

 a vertical diameter of 36 millimeters and a transverse 

 diameter of 32 millimeters. The superior border of its 

 exit is somewhat in advance of its inferior, so that it looks 

 upward and backward. 



Fig. 8.— Occipital region of skull of Triceratops flabel- 

 latus (type), No. 1821, Yale Museum, as seen 

 obliquely from beneath, in front, pfp, Pillar for 

 support of parietals and postfrontals; so, supra- 

 occipital; ex, exoccipital process broken away; 

 as, alisphenoid; ab, alisphenoid buttress for post- 

 frontal; plf, postfrontal; oc, occipital condyle; 60, 

 basioccipital process; bs, basisphenoid; pt, surface 

 on, basiphenoid process for contact with ptery- 

 goid; xii, foramen for twelfth nerve; x-xi, fora- 

 men for exit of tenth and eleventh nerves; flp, 

 foramen lacerum posterius; earn, internal audi- 

 tory meatus?; ca, foramen for carotid artery; fo, 

 foramen ovale; z, undetermined foramen; pf, fora- 

 mina entering pituitary fossa; sf, sphenoidal fis- 

 sure; of optic foramen; fr, foramen rotundum; 

 ff, small foramina entering brain cavity just back 

 of olfactory lobe. One-eighth natural size. 



THE SPHENOIDAL SEGMENT. 



The basisphenoid is firmly coossified with the alisphe- 

 noids. The basisphenoidal processes are produced some- 

 what beneath the basioccipital processes and present in 

 front at their extremities rugose surfaces for contact with 

 the pterygoids. Anteriorly and superiorly the basisphe- 

 noid is compressed, and forms a stout median interorbi- 

 tal septum. The external opening of the middle eusta- 

 chian canal is situated between and at the base of the 

 basisphenoidal processes, as shown at mec in fig. 9 and in PI. XLVL It is entirely within the 

 basisphenoid instead of being situated between that bone and the basioccipital, as in the croco- 

 dile. Two large foramina, situated one on either side of the skull at the bases of the basisphe- 

 noidal processes, pierce the basisphenoid and enter the brain case near the base of the olfactory 



MON XLIX — 07 2 



