210 DINOSAURS OF NORTH AMERICA. 



a large sinus, extending above the brain ease into the cavities of the 

 horn cores. This foramen has not before been observed in dinosaurs. 



THE POSTERIOR CREST. 



The enormous posterior crest is formed mainly by the parietals, which 

 meet the postfrontals immediately behind the horn cores. The poste- 

 rior margin is protected by a series of special ossifications, which in 

 life had a thick horny covering. These peculiar ossicles, which extend 

 around the whole crest, the writer has called the epoccipital bones (PI. 

 LX, figs. 1-3, e, and PI. LXI, fig. S, e). In old animals they are firmly 

 coossified with the bones on which they rest. 



The lateral portions of the crest are formed by the squamosals, 

 which meet the parietals in an open suture. Anteriorly they join the 

 postfroutal elements, which form the base of the horn core, and later- 

 ally they unite with the jugals. The suprateinporal fosste lie betweeu 

 the squamosals and the parietals. 



BASE OF SKULL. 



The base of the skull has been modified in conformity with its upper 

 surface. The basioccipital is especially massive, and stroug at every 

 point. The occipital condyle is very large, and its articular face nearly 

 spherical, indicating great freedom of motion. The basioccipital 

 processes are short and stout. The basipterygoid processes are longer 

 and less robust. 



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- 

 occipitals. They form about three-fourths of the occipital condyle, as 

 in some of the chameleons. The supraoccipital is very small, and its 

 external surface is excavated into deep cavities. It is coossified late with 

 the parietals above and with the exoccipitals on the sides (PI. LX, fig. 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 quadratojugal is a solid, compressed bone, uniting the quadrate 

 with the large, descending process of the jugal. In the genus Tricera- 

 tops the quadratojugal does not unite with the squamosal. In Cera- 

 tops, which includes some of the smaller, less specialized forms of the 

 family, the squamosal is firmly united to the quadratojugal by suture. 



The quadratojugal 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 (PI. LX, fig. 1). This greatly strengthens the 

 (•enter of the skull, which supports the horn cores, aud also tends to 

 modify materially the elements of the palate below. The pterygoids, 



