EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



533 



thus being a progressive advance beyond the M. tri- 

 gonoceras condition. The hypocone of m' is tubercu- 

 late, with a narrow crest extending off toward the 

 metacone; this tooth exhibits a sessile "crochet" 

 and "antecrochet." 



Additional measurements of the type of " B. ingens" 



Millimeters 



Summit of occiput to tip of nasals 755 



Extreme spread of horns 490 



Free length of nasals 125 



Width of nasals 125 



Molar-premolar series, estimated 428 



True molar series 265 



Dentition. — The incisors either consist of extremely 

 reduced crowns in depressed alveoli, resembling the 

 vestiges seen in M. trigonoceras, or are entirely 

 obsolete. Of the male skulls in the American Mu- 

 seum No. 1066 shows two vestigial alveoli and No. 505 

 shows one small alveolus; similarly, in the National 

 Museum No. 4291 shows two alveoli partly closed. 

 In one of the female skulls (Am. Mus. 1066) there 

 is a single vestigial incisor on one side, lying in its 

 alveolus below the surface of the jaw. 



Canines: Contrasting with this atrophy is the 

 hypertrophy of the canines, which are readily dis- 

 tinguished from those in members of the Bronto- 

 therium phylum by their long and pointed crowns, 

 circular to suboval in section. Almost completely 

 encircling the base of the crown is a prominent 

 cingulum; this is a very exceptional and distinctive 

 character; in the females the cingulum is perhaps 

 somewhat less prominent. In the males the largest 

 canines measure about 70 millimeters; in the females 

 the canines measure about 40 millimeters. The chief 

 distinction of the female tusks is their much more 

 slender and pointed character. 



Premolars: The persistence of the first premolars in 

 both jaws is a general characteristic of this species, 

 although not observed in the specimen in the Field 

 Museum. A peculiarity of p' is the presence of a 

 distinct tetartocone, which is even stronger in "B. 

 ingens " than in M. trigonoceras. In the succeeding 

 premolars, p^-p*, the tetartocones are more or less 

 well defined and distinct from the deuterocones, either 

 connected by a low ridge or altogether separate, the 

 proportion between the size of the deuterocone and 

 tetartocone being as 5 to 3. This acceleration of the 

 tetartocone is a progressive character; it is less ad- 

 vanced in the American Museum skulls than in the 

 Yale Museum type of "B. ingens," which was prob- 

 ably found on a very high geologic level. As a rule 

 the tetartocones increase in distinctness as we pass 

 backward from p' to p^, but as in all other titano- 

 theres the tetartocone in p* is generally less distinct. 

 A mesostyle ridge is faintly developed on p* in some 

 specimens of Menodus. It is best developed in cer- 

 tain specimens of Brontofherium and Megacerops in 

 which p* is more molariform than in Menodus. 



Both molars and premolars have deep pits or 

 "medifossettes" in the midvalley, just internal to the 

 ectoloph; this "fossette" is bounded internally 

 by folds homologous with the "antecrochet" and 

 "crochet" of rhinoceros molars. The molars and to 

 a less extent the premolars are proportionately nar- 

 rower and longer than in the brachycephalic phyla. 



Molars: The "fossette" just mentioned is correlated 

 with the vertical elongation of the ectoloph, which 

 now measures 80 millimeters in height, while the pro- 

 tocone measures only 28 millimeters; the outer wall of 

 the tooth is thus between two and three times as high 

 as the inner wall. As in the preceding species of this 

 phylum, the internal cusps also of the grinding teeth 

 have steep slopes. 



The hypocone of m^ shows individual variability: 

 it is either large and distinct (Am. Mus. 1067), or 

 small and distinct (Nat. Mus. 4291), or confluent 

 with the cingulum (Am. Mus. 1066). 



The cingulum is certainly the most conspicuous 

 and distinctive character of the grinding teeth; not 

 only does it encircle the canine, but it is sharply 

 defined upon the outer faces of the entire superior 

 and inferior premolar-molar series (Pis. CXXXVI, 

 CXXXVII, CXXXIX). The superior premolars are 

 readily distinguished by the broad shelf-like internal 

 cingulum, unlike that in any other phylum. The 

 superior molars are also cingulate upon the inner 

 sides. The only part of the entire dental series which 

 entirely lacks the cingulum is the inner side of the 

 inferior premolar-molar series, where no titanothere 

 displays a cingulum. 



The grinding series as a whole, therefore, is distin- 

 guished by its great length, by the proportionate 

 length of the individual teeth, the anteroposterior 

 slightly exceeding the transverse diameter (in the 

 molars), and by the pronounced development of the 

 cingulum. 



Skull. — The chief distinction of the skull Ues in its 

 proportions; it is relatively long and narrow, the 

 average ratio of two male skulls being, length 773 

 millimeters, breadth 545 millimeters, and consequent 

 index 69, whUe in the type of Brontops rohustus the 

 ratio is 765 to 667, and the index is 87. In all the 

 skulls the nasals are large and quadrate, measuring 

 from 127 to 175 millimeters in length and from 125 

 to 145 and more in breadth. As in M. trigonoceras, 

 advancing age is accompanied by a rugose develop- 

 ment of the extremities, with a lateral expansion of the 

 tips and a deepening of the median cleft. Other 

 growth characters of the skull are the increasing length 

 of the horns from 150 to 290 millimeters, the increasing 

 rugosity and breadth of the occiput, the widening of 

 the buccal expansions of the zygomata. These buccal 

 expansions, however, remain limited in extent and do 

 not involve a large part of the zygomatic arch as in 

 Brontotherium; immediately below and slightly anterior 

 to the buccal expansion is a very characteristic, 



