38 
Mandible. {Figures 3 and 20.) The mandible of Edmontosaurus 
is extremely long. The converging rami meet in a short horizontal 
symphysis and are embraced in front by the unpaired pre- 
dentary bone. The rami are deepest at their midlength, and attain 
their maximum breadth at about two-thirds of their length from 
the front where the conspicuously high coronoid process is developed. 
In lateral aspect they have a nearly straight or slightly undulating inferior 
outline. As seen from above they are bow-shaped with an inward bend 
at midlength, and an incurve at either end abrupt in front where they 
meet, and less so behind where they are some distance apart. Each 
ramus consists of a dentary, which is edentulous for about three-eighths 
of its length in front, a surangular, an angular, a splenial, an articular, 
and possibly a prearticular, but whether this element is certainly present 
had not been ascertained. The dentary forms the greater part of the 
Figure 20. Left mandibular ramus of Edmontosaurus, Cat. No. 2289, viewed from 
the inner side; ^ natural size. An, angular; Ar, articular; Cp, coronoid process; Dn, 
dentary; mf, mandibular fossa; mg, Meckelian groove; Sa, surangular; Sp, splenial. 
ramus. Of the other comparatively small elements, composing the hinder 
end of the ramus, the surangular is robust and much the largest, the 
angular is long and slender, and the articular the smallest. The splenial 
and the angular together make up the greater part of the inner surface 
of the ramus posteriorly. The surangular supplies the lower and outer 
surfaces at the hinder end. The articular lies above the surangular 
between it and the splenial and to a limited extent is exposed externally 
above the surangular. The dental magazine is nearer the posterior than 
the anterior end of the ramus, and is for the most part in the dentary ’s 
posterior half-length. The edentulous portion of the dentary is only 
slightly less than one-third of the length of the ramus. 
The cotylus by means of which the mandible articulates with the 
quadrate is far back at a very short distance in advance of the angle .of 
the jaw. It is provided principally by the surangular, but the articular 
also, to a small extent, enters interno-superiorly into its formation. The 
mandibular fossa is of large size and is confluent in front with the Meckelian 
groove which latter extends forward inferiorly on the inner surface of the 
dentary. The fossa is enclosed externally by the coronoid process of the 
dentary and the surangular, inferiorly by the dentary and the surangular, 
and internally by the dentary, splenial and angular. Antero-internally 
it opens inward through a long, narrow vacuity (internal mandibular 
foramen) occurring between the angular below, and the splenial and 
dentary above. 
