EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



495 



of B. dispar also show more or less transition to this 

 transversely flattened type of horn. 



Correlated with the general brachycephaly, the 

 palate is broader and the grinding series more widely 

 arched. Similarly we observe great breadth in the 

 postglenoid and paroccipital processes. It is to be 

 noted that the postglenoid and post-tympanic proc- 

 esses unite late in life (Am. Mus. .1069), whereas in 

 the contemporaneous species of Brontotherium {B. 

 gigas and B. curtum) these processes unite early in life. 

 The vomerine ridge is prominent, and there is a down- 

 ward continuation, or septum, of the vomer tending 

 to unite with the palatine and thus divide the posterior 

 nares. This feature is observed also occasionally 

 in B. dispar. Rugosities for the attachment of the 

 recti capitis antici majores muscles are located at the 

 junction of the basioccipital and basisphenoid (type 

 skull and Am. Mus. 1069), another Brontops character. 

 The external bridge over the infraorbital foramen is 

 relatively narrow in contrast with its more primitive 

 width, correlated with the longer face in A. marshi. 

 The antorbital malar ridge is less rounded than in 

 B. gigas and its congeners. There is a pair of small 

 protuberances (k, k) on either side of the median line 

 of the upper portion of the occiput, which very prob- 

 ably were situated between the insertion areas of 

 the median ligamentum nuchae and of the paired 

 semispinalis capitis muscles. These protuberances 

 are also observed in A. marsJii, B. gigas, and B. 

 curtum. The great strength of the occipital pillars 

 and the broadly rugose flare of their summits contrast 

 with the narrow ridges observed in B. dispar. 



The hyoid elements, represented in Figure 425, 

 include the stylohyals, 265 millimeters in length, 

 which are expanded superiorly and exhibit a long, 

 flattened shaft and a slight distal expansion terminat- 

 ing in cartUage; the ceratohyals, 80 millimeters in 

 length, expanded at both extremities. The epihyals 

 were probably almost vestigial, as in other peris- 

 sodactyls. In the horse they consist of a small 

 nodule of bone embedded in the cartilage that con- 

 nects the ceratohyal with the stylohyal. 



Dentition. — The variability in the upper incisors 

 has already been mentioned. The formula is probably 

 If^. The incisors exhibit the strongly convex, 

 smoothly rounded crowns so characteristic of Teleodus 

 avus, B. hracTiycepTialus , A. marshi, and B. dispar. 

 The canines are quite distinctive of this species. In 

 the males they are large and robust, recurved, pointed, 

 and some specimens (Am. Mus. 1069) possess a 

 strong posterior cingulum, thus being readily distin- 

 guishable from the canines of the contemporary Meno- 

 dus and Brontotherium. The diastema behind the 

 canine of the Yale type skull is unusual, although it 

 is seen occasionally in the earlier Brontops hrachycepha- 

 lus and appears to be a somewhat distinctive specific 

 character, since it is also observed in the two male 

 skulls (Am. Mus. 1083, 1069). The premolars are 



highly distinctive in their retarded stage of evolution; 

 p^ exhibits a rudimentary postero-internal cusp; the 

 tetartocones of the succeeding p^"^ are rather feebly 

 developed; in p* the tetartocone is a mere spur; the 

 external cingula are barely indicated, and the internal 

 cingula of the premolars are rounded and crenulate. 

 In the molars also the cingula are irregularly de- 

 veloped. The hypocone of m^ is variable in form, 

 either cingulate (Am. Mus. 1083) or a small, acute 

 cusp (Am. Mus. 1069). A reversional feature of 

 interest is a crenulate crest or abortive metaloph 

 connected with the hypocone, recalling the similar 

 structure in B. irachycephalus. Skull Am. Mus. 

 1069 exhibits on m^ a well-defined "crochet" and 

 " antecrochet, " and also a minute "crista." As 



Figure 422. — Skull of Brontops rohusius 



Top view. One-ninth natural size. Am. Mus. 1069; resembles 

 the type of B. robustus in its extreme brachycephaly. The 

 horns are shorter and thiclier at the tips. The midparietal 

 opening is seen also in certain skulls of B. dispar. 



observed above, the grinding series tend to rise 

 anteriorly. 



Additional details of the type sTcull. — In reference to 

 the characters noted above, it is important to observe 

 that the top of the type skull is somewhat crushed 

 from front to back, increasing the concavity along 

 the top line and the spread of the vertex. The 

 grinding series are in the ninth stage of wear. It is 

 in this skull that we observe extraordinary breadth 

 and relative shortness, extremely robust zygomata, 

 horns extremely far forward and oval in transverse 

 section, well overhanging the abbreviated nasals, and 

 the arching character of the opposite grinding series. 

 The chief measurements are as follows: Top of 

 occiput to tip of nasals 635 millimeters, transverse 



