394 R. S. Lull — Cranial Musculature in Dinosaurs. 



topsia representing the space between the median and lateral 

 elements of the parietal region which has been constricted off 

 from the present snpratemporal fossae by a union of the parie- 

 tal elements at their anterior end. Evidence in favor of this 

 view is seen in the type specimen of Torosaurus latus Marsh, 

 here figured (fig. 4), in which a distinct suture may be seen on 

 either side leading from the parietal fenestra forward to the 

 supratemporal fossa and representing the line of final closure 

 of the bony bridge separating the two openings. Torosaurus 

 gladius Marsh has a faint indication of the same suture, thus 

 exhibiting a greater degree of specialization than its ally. 

 Centrosaurus apertus Lambe from the Judith river beds also 

 has indications of the same suture (fig. 5), while in the specimen 

 of Monoclonius crassus Cope from the same horizon the suture 

 probably existed if indeed the closure was complete. 



Fig. 5. Centroscnirus cqiertus Lambe ; modified from Lambe. 



The bone is fractured at this region, unfortunately in the 

 only known specimen, so that this question cannot be 

 decided. In Ceratops canadensis Lambe, also from the 

 Judith river, the squamosal is relatively much larger than 

 in most of its contemporaries and the lateral element of the 

 parietal, the only part preserved, is a slender bar of bone 

 which did not unite with the median element forward (fig. 

 7). It is probable, however, that the shutting off of the 

 fenestra from the supratemporal fossa was effected by the 

 widening of the anterior portion of the median parietal ele- 

 ment to meet the squamosal. In Triceratojps the final closure 

 of the parietal fenestrse is effected by a continuous growth of 

 bone over the entire parietal region of the frill (fig. 6). This 

 bone is, however, extremely thin in what would be the anterior 



