or horned Dinosaurs, of North America. 171 



The foramen magnum is very small, scarcely one-half the 

 diameter of the occipital condyle. The brain-cavity is especially 

 diminutive, smaller in proportion to the skull than in any 

 other known reptile. 



The exoccipitals are also robust, and firmly coossified with 

 the basi-occipital. They form about three-fourths of the 

 occipital condyle, as in some of the Chameleons. The supra- 

 occipital is very small, and its external surface is excavated 

 into deep cavities. It is coossified late with the parietals 

 fibove, and with the exoccipitals on the sides (Plate I, 

 figure 2). 



The quadrate is robust, and its head much compressed. The 

 latter is held firmly in a deep groove of the squamosal. The 

 anterior wing of the quadrate is large and thin, and closely 

 united with the broad blade of the pterygoid. 



The quadrato-jugal is a solid, compressed bone, uniting the 

 quadrate with the large descending process of the jugal. In 

 the genus Trieeratops, the quadrato-jugal does not unite with 

 the squamosal. In Oeratops, which includes some of the 

 smaller, less specialized forms of the family, the squamosal is 

 firmly united to the quadrato-jugal by suture. 



The quadrato-jugal arch in this group is strong, and curves 

 upward, the jugal uniting with the maxillary, not at its 

 posterior extremity, but at its upper surface (Plate I, figure 1). 

 This greatly strengthens the centre of the skull which supports 

 the horn-cores, and also tends to modify materially the elements 

 of the palate below. The pterygoids, in addition to their 

 strong union with the quadrate, send outward a branch, which 

 curves around the end of the maxillary. 



The palatine bones are much smaller than the pterygoids. 

 They are vertical, curved plates, outside and in front of the 

 pterygoids, and uniting firmly with the maxillaries. The 

 vomers join the pterygoids in front, where they appear as thin 

 hones, closely applied to each other. 



The transverse bones give some support to the maxillaries, 

 which are further strengthened by close union with the 

 pterygoids. They meet the pterygoids behind, and the pala- 

 tines in front. 



The lower jaw shows no specialization of great importance, 

 with the exception of the pre-dentary bone already described. 

 There is, however, a very massive coronoid process rising from 

 the posterior part of the dentary (Plate I, figure 1) The 

 articular, angular, and surangular bones, are all short and strong, 

 but the splenial is very long and slender, extending to the 

 pre-dentary. The angle of the lower jaw projects but little 

 behind the quadrate. 



