[lambe] cretaceous GENUS STEGOCERAS 33 



sauridœ the protective covering of scutes hides the upper inventing 

 bones of the skull so that until material representing the members of 

 this group has been further studied a full comparison of the skull in 

 the two families cannot be made. 



Stegoceras differs from other known forms of armoured dinosaurs 

 in the manner in which the head is protected. A marvellous thicken- 

 ing of the elements above the brain-case and a more moderate streng- 

 thening of the lateral elements resulted in modifications affecting the 

 proportions of the bones as well as their mode of suturai union. Where- 

 as in the dinosauria generally the majority of the bones of the upper 

 surface of the skull are moderately thin and many of them have squa- 

 mous sutures, in Stegoceras the thickness of the bones usually precludes 

 an overlapping contact and results in a simplified contour with a re- 

 duction to a minimum of prolongations which if they do occur are 

 heavy and robust as seen in the forwardly directed spur of the squa- 

 mosal beneath the postorbital. The upper bones of the skull in 

 Stegoceras show a tendency to become prismatic, meeting in vertical 

 or nearly vertical suturai planes or facets which are generally marked 

 by numerous irregular vertical ridges and grooves. The thick bones 

 meet and support each other after the manner of the ice blocks in an 

 Eskimo hut or the hewn members of a stone arch. 



The thickness of the bones (parieto-frontal mass) above the 

 brain-cavity seems quite disproportionate to the size of the cavity 

 itself but is thoroughly in accord with the surprising weight of bone 

 borne by most of the Cretaceous armoured dinosaurs in the form of 

 heavy plates and spines on the back and tail. 



In Stegoceras there are certain features in the form and com- 

 position of the skull which suggest a relationship to the Stegosaurus 

 type. These resemblances are seen in the posterior breadth of the 

 cranium, the well separated supratemporal fossae, and the large size of 

 the laterotemporal fossae and orbits, the last looking outward from 

 beneath overhanging rims. In both genera supra-orbital bones are 

 developed, two pairs in Stegosaurus but apparently only one in 

 Stegoceras. 



The differences marking the divergence of Stegoceras from the 

 Stegosaurus type are seen, in the former, in the abbreviation of the 

 skull, in the union of the parietals and frontals in a solid mass of bone 

 of extraordinary thickness, and in the general thickening of the 

 remaining upper cranial elements accompanied by a horizontal expan- 

 sion of the bones of the supratemporal arcade, reducing the size of the 

 supratemporal fossae, or entirely closing them, and causing the pos- 

 terior border and the hinder part of the lateral borders to overhang. 



Sec. IV, Sig. 3 



