458 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



Features of the jaws. — As most of the jaws found 

 were entirely dissociated from the skulls, it is difficult 

 to determine the generic and even more so the spe- 

 cific references of many separate jaws. Observation 

 should be directed first to the prevailing dolichoce- 

 phalic or brachycephalic proportions, to the shallow- 

 ness or depth of the ramus, to the slenderness or 

 massiveness of the angular region, and especially to 

 the depth or shallowness of the chin region. In 

 general, members of the menodontine group have a 

 deep symphyseal or chin region, whereas members of 

 the brontotheriine group have a shallow chin. 



The generic diagnosis of remains that include a full 

 series of teeth is relatively simple. Specific diagnosis is 

 partly dependent on size. In order to associate a 

 lower jaw with a skull the measurement should be 



Characters of the jaw that distinguish members qf the menodontine group from members of the brontotheriine group 



taken from the glenoid cavity to the front of the 

 superior canine and from the mandibular condyle to 

 the posterior face of the inferior canine. If all the 

 other characters are properly determined and the 

 progressive stages of the superior and inferior pre- 

 molars correspond, this method of associating the jaws 

 with the skulls is reliable. This method, however, 

 can be used to advantage only on skulls and j aws that 

 are not distorted by crushing. Usually the most 

 practicable measurements are the following: 



Upper: Front face of canine to middle of posterior fossa 



of m^ (for hypoconulid of ma). 

 Lower: Rear face of canine to tip of hypoconulid of ma. 



The following table shows the chief characters of 

 the jaw that distinguish the members of the two 

 groups : 



Horizontal ramus 



Symphyseal region in 

 side view below men- 

 tal foramen. 



Region of angle 



Incisors 



Pi in fully adult jaws- 

 Diastema in front of pi 

 Canines __ 



External cingulum of 

 canines, premolars, 

 molars. 



Upward flexure of 

 premolar series. 



Menodontine group 



Deep 



Convex.. 



Barely pro- 

 duced. 



I3 



Present 

 Present 

 Slender. 



Not sharply 

 defined. 



Shallow 



Very shallow.. 



Usually produced 

 downward. 



Usually present 



Present, wide -. 



Slender to short, stout. 



Present in early types; 

 lost in B. robustus. 



Slight or moderate. 



Shallow. 

 Shallow. 



? Produced, 

 truncate. 



Present 



Present 



Slender 



to short, 



stout. 

 Intermedi- 



ate. 



Slight or 

 moderate. 



Allops 



Intermediate -. 



Intermediate to con- 

 vex. 



Intermediate - 



l2_l.— -. 



Present or absent 



Absent 



Conic to compressed 

 anteroposteriorly. 



Present-- - 



Very slight 



Deep... 

 Fuller.. 



Produced backward 

 and downward into 

 a convex elbow. Pos- 

 terior border obli- 

 que. 



lo (typically) 



Present or absent 



Absent 



Conic - 



Strongly marked- 



Brontotheriine group 



Short, massive. 



Broad, posterior 

 border vertical. 



1 lo 



?Absent.. 

 7 Absent. 

 (?) 



Brontotberium 



Massive, deep poste- 

 riorly. 

 Very shallow. 



Broad, posterior border 

 often vertical. 



Present or absent. 



Absent. 



SwoUen at base with 



massive p s t e r i or 



cingulum. 

 Absent. 



Typically pronounced. 



