EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



479 



Teleodus primitivus (Lambe). Known from a jaw found in 

 the Cyp ress Hills, containing three lower incisor teeth, of which 

 i2 is the largest and ii the smallest. 



Brontops brachycephalus (Osborn). Very abundant in the 

 lower beds, extending from the lower into the middle levels and 

 passing into transitional forms in the upper levels. Readily 

 distinguished by the broad, rounded skull, and very simple 

 premolar teeth. 



Brontops dispar Marsh. Very abundant and characteristic of 

 the middle levels. Transitional in structure between B. 

 brachycephalus and B. robustus. 



Brontops robustus Marsh. An enormous animal characteristic 

 of the lower levels of the upper beds. It presents in an extreme 

 degree the progressive characters of this genus — great width of 



the skull combined with decided abbreviation of the horns as 

 well as of the nasals and surprisingly backward development of 

 the premolar teeth. 



Diploclonus bicornutus (Osborn). Closely related to Brontops 

 dispar, from which it differs in its longer, narrower nasals and 

 internal hornlets on the horns. Geologic level unknown, prob- 

 ably the lower part of the middle beds. 



Diploclonus lyleri (Lull). Intermediate between D. bicornutus 

 and D. amplus. Brachycephalic, with shortened nasals and 

 well-developed internal hornlets. Geologic level propably mid- 

 dle beds. 



Diploclonus amplus Marsh. Extremely brachycephalic, with 

 short nasals; divergent horns with steep connecting crest. 

 Probably from the upper beds. 



Conspectus of characters of jaw in the menodontine group 



MEASUREMENTS OF THE BRONTOPS SERIES 



The species of the genus Brontops, like the species of 

 Menodus, constitute a closely graded ascending series 

 beginning in the lowest and running up into the latest 

 beds of the Titanotherium zone. The range in the 

 chief measurements of the successive stages may 

 be summarized as follows: 



Measurements of Brontops, in millimeters 



Pi-m' 



Pi-p* 



Mi-m3 



Pmx to condyles 

 Zygomatic index 



Nasal length 



Horn length 



340-376 

 132-161 

 215-236 

 743-843 

 77-87 

 52-90 

 130-210 



310-345 

 123-145 

 195-215 

 660-687 

 78-87 

 85-90 

 155-198 



B. brachy- 

 cephalus, cf 



265-297 

 101-123 

 160-180 

 580-610 



72-?82 

 60-85 



59-135 



Hence the series of measurements exhibits small 

 gaps between B. brachycephalus and B. dispar in the 

 measurements p'-m', m'-m', in the basilar length 

 (pmx to condyles), and in the length of the horns; but 

 the two species are continuous or overlap in respect to 

 the length of the premolar series, in the zygomatic 

 index, and in the nasal length. On the other hand 

 B. dispar is continuous with or overlaps B. roiustus 

 in the measurements p'-m', p'-p*, m'-m^, as well as 

 in the zygomatic index, nasal length, and length of 

 horns; but there is a sharp gap between the two species 

 in the basilar length, B. roiustus being a far larger 

 type. 



