20 ILLINOIS BIOLOGICAL MONOGRAPHS [190 



sensory canals in the bone with openings at various points. The outer 

 surface is smooth except that it has a narrow ridge at its postero-lateral 

 edge which is located slightly anterior to the spines. The notch or pit 

 formed by the curve is filled with a thin wavy sheet of bone which 

 slightly overlaps the caudal edge of the shank of the hyomandibular. 

 Slightly dorso-anterior to the ridge is a small roughened area for the 

 articulation of the suborbital stay. 



Mandible. — The mandible is made up of three bones, the dent- 

 ary, articular and the angular. A broad rounded ridge runs from 

 slightly anterior to the head of the articular nearly to the anterior end 

 of the dentary. On the inner surface of the dentary is a cavity within 

 the bone corresponding to the ridge on the exterior; the articular has 

 no cavity but a groove which corresponds to the ridge. In the bone 

 beneath the ridge there is a latero-sensory canal which extends from 

 the anterior end of the dentary to the posterior end of the articular. 

 The dentary is a long bone with two long processes at the posterior end, 

 forming a V-shaped angle which fits over the anterior process of the 

 articular for its articulation with that bone. The lower surface is 

 broadened by a mesial extension of a plate-like bone forming the ventral 

 surface. Two kinds of teeth are present on the dentary, an inner row 

 of the large canine type and several irregular rows of the villiform 

 variety. 



Articular. — The articular is united to the dentary by several bands 

 where it fits into the V-shaped angle for the union of the two bones. This 

 bone has a large facet for articulation with the quadrate. Here the bone 

 is quite broad and gives off three processes. Slightly anterior to the facet 

 it gives off a dorsal anterior process which unites with the dorsal process 

 of the dentary by a cartilage. Immediately ventrad to this, and anterior 

 to the facet, it gives off a long process, the anterior end of which fits 

 into the V-shaped angle of the dentary. On the ventro-anterior edge it 

 gives off the third process which fits over the ventral process of the 

 dentary. There is a groove on the inner surface of the anterior and 

 ventral processes and a deep pit on the antero-ventral edge of the facet. 

 Ventro-posterior to the facet is located the angular bone which serves 

 as an attachment for some of the mandibular muscles to the quadrate 

 and also to the intraopereular. 



