EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



559 



Other features distinguishing B. leidyi from A. 

 walcoUi are no median vertical ridge on occiput; no 

 lateral projections from midtemporal crests; great 

 breadth opposite supraorbital processes; horns slightly 

 more anterior to orbit; no lacrimal knob; broad con- 

 tact of postglenoid and post-tympanic processes. 



Oiservations on the measurements of Brontotherium 

 leidyi. — The type and hypotype specimens agree fairly 

 well in measurements, and this primitive species is very 

 clearly defined. The zygomatic index is extremely 

 low, owing to the nonexpansion of the zygomata; as 

 compared with B. platyceras the nasals are about three 

 times as long, while the elevation of the horns above the 

 nasals is only about one-fourth as great. 



The type skull (Nat. Mus. 4249) is in the beginning 

 of the ninth growth stage. The zygomata, although 

 much crushed, give evidence of a 

 sudden flattening and buccal expan- 

 sion posteriorly, similar to that which 

 we observe on a larger scale in B. 

 gigas. The occiput shows narrow 

 lateral pillars; it was deeply cleft 

 above. The nasals are remarkably 

 long, narrowing anteriorly, and de- 

 curved. The bridge over the infra- 

 orbital foramen is rather broad. The 

 infraorbital foramen is partly exposed 

 on the side of the face. The horns 

 rise a very short distance above the 

 vertex and, unlike those in A. walcotii, 

 are markedly recurved and broadly 

 oval at the top. Judging by the 

 alveoli, there is a pair of large lateral 

 incisors with a vestigial alveolus for a 

 median incisor on the right side. The 

 dental formula is thus I^^, P^. The 

 premolars have rounded inner and 

 outer surfaces and vestigial or even 

 no trace of cingula and in this respect 

 resemble those in certain specimens 

 of upper Titanotherium zone species 

 of this series. The tetartocones are 

 strongly developed in p^, p^, p*, occu- 

 pying about two-fifths of the inner 

 surface of the crown in p^, p^ and 

 being large and prominent even on p*. 

 The medifossettes are much deeper 

 than in Brontops hrachycephalus. The 

 hypocone is cingulate on m^ and 

 presents a very strong crest or aborted metaloph 

 extending in toward the metacone; m^ shows the 

 so-called antecrochet and crochet. 



The beautifully preserved specimen in the Carnegie 

 Museum (No. 93, Pis. CLXI-CLXV, fig. 462) differs 

 from the National Museum type in the following 

 points: (1) The median incisor is persistent but 

 reduced; (2) the premolars have sessile, crenulate 

 internal cingula; (3) the tetartocones are set even 



farther in toward the middle of the crown; (4) the 

 preorbital malar bridge is narrower and rounder. It 

 is thus in every respect a characteristic Brontotherium. 

 It shows very large, flat-crowned, functional incisors; 

 exceptionally heavy, blunt, posteriorly cingulate 

 canines in the upper and lower jaws; p^ preserved 

 but small, tetartocones prominent, well rounded; 

 cupped and crenulate hypoconulid on ms, occipital 



Figure 462. — Lower jaws of Brontotherium leidyi 

 A, Carnegie Mus. 93; ramus crushed vertically but teeth well preserved, chin shallow, angle not produced, canine 

 very stout, no diastema in front of pi, grinding teeth without external cingula. B, Am. Mus. 516; provisioually 

 referred to B. leidyi on account of the near agreement in measurements. Incisors stout, canine short and 

 swollen, a small diastema in front of pi, cheek teeth almost lacking external cingula, premolar series curved 

 upward anteriorly. One-fifth natural size. 



knobs (PL CLXV), inconspicuous infraorbital fora- 

 men and rounded malar bridge, nasals thin except 

 along median inferior ridge, chin sloping, coronoid 

 hooked. Taken in connection with the type it proves 

 that Brontotherium must have acquired its generic 

 characters very early, especially since both these 

 specimens are from the lower zone. 



Jaw of B. leidyi. — The jaw in this stage of evolution 

 is represented by a specimen (Am. Mus. 516) which 



