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 fiUed 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 ty^e 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 intraopercular. 



