41 
Temporalis (Figures 1-6, 8, tm.). This was the principal agent in the 
closing of the ceratopsian jaws. In Protoceratops (Figure 1) it was a 
broad, fan-like sheet, covering almost the entire parietal portion of the 
crest, passing forward through the supratemporal opening, and attaching 
to the inner side of the coronoid process. The backward extension of the 
crest in Chasmosaurus (Figure 2) greatly lengthened this muscle, with 
corresponding increase in contractile power, but there was no essential 
change from the condition in Protoceratops. Styracosaurns (Figure 3) and 
Centrosaurus (Figure 4) have the origin area more restricted, owing to the 
smaller size of the crest and its fenestra. The area of the muscle is clearly 
defined in Styracosaurus ; on the medial side there is an unobstructed 
Figure 4. Centrosaurus flex • 
vs (Brown), Geol. Surv.. 
Canada, No. 348, dorsal 
view of crest, with, tem- 
poralis muscle (tm.) re- 
stored, also some correc- 
tion for distortion; X 
Figure 3. Styracosaurus albertensis Lambe, holo- 
type, Geol. Surv., Canada, No. 344, dorsal view of 
crest with temporalis muscle (tm.) restored; 
X • 
channel from fenestra to supratemporal opening, but more laterally the 
muscle is cut off abruptly along a transverse line. Similar conditions appear 
in Centrosaurus and the somewhat later Anchiceratops (Figue 5), but here 
the posterior portion of the muscle appears to have been thin and mem- 
branous. Finally, in Triceratops (Figure 6), the sharp line of origin first 
seen in Styracosaurus has become the posterior margin of the supratemporal 
opening, indicating that the temporalis muscle was restricted to that 
vacuity. 
It is evident, therefore, that the crest or frill of the ceratopsians was 
primarily developed to extend the area of origin, and the size, of the tem- 
poralis muscle. This seems to have been the entire function of the crest 
in Protoceratops. In Chasmosaurus and Torosaurus this development 
reached its maxima, with the crest retaining large vacuities to permit 
expansion of the muscle mass. The blunt marginal serrations and spikes 
of the Chasmosaurus crest seem inadequate concessions to defense. Con- 
temporaries of this genus, however, had already developed the defensive 
