456 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



STAGES OF WEAR OF THE ADULT GRINDING TEETH 



Seven early and adolescent stages in the ontogeny 

 of the dentition have been described above. In the 

 eighth and ninth stages (adolescent) the deciduous 

 premolars (dp^ dp^ dp'') are gradually replaced by 

 the permanent premolars p^, p^, p*. In the tenth 

 ontogenetic stage (X) of the young adult the internal 

 cusps of the second and third but not the fourth 

 premolars are beginning to show the dentine through 

 the enamel surface; the last molar is but little worn. 

 In the very aged fifteenth ontogenetic stage (XV) 



XII. Twelfth ontogenetic stage: Broniops robustus 

 Yale Mus. 12048 (type). 

 X. Tenth ontogenetic stage: Alloys crassicornis, 

 Nat. Mus. 4289 (type). 



AGE AND OTHER CHARACTERS COMMON TO BOTH SEXES 

 OF TITANOTHERES OF ALL STRATIGRAPHIC LEVELS 



Age characters. — The age characters are naturally 

 much more conspicuous in males than in females. In 

 both sexes the adults of one geologic generation follow 

 the general law of anticipating the advanced muta- 

 tions or specific stages reached by adults of higher 

 geologic levels. Thus the variability of a tooth on a 



Figure 388. — Skull contours showing extreme divergence between Menodus giganteus (A), a final term of 

 the menodontine series, and Brontotherium platyceras (B) , a final term of the brontotheriine series 



In Menodus the opposite borders of the cranial roof diverge anteriorly, the horns are short and trihedral, the zygomata but little expanded, 

 and the occiput not greatly produced backward. In Brontotherium the opposite borders of the cranial roof are nearly parallel, the 

 horns very long and flattened, the zygomata widely expanded, and the occiput greatly produced backward. One-tenth natural size. 



nearly the whole enamel surface of the crowns of 

 p'-m' inclusive has been worn away, so that the 

 dentine is very widely exposed; in m^ both the pro- 

 tocones and hypocones are much worn. The inter- 

 vening stages show intermediate conditions, as follows 

 (fig. 387) : 



XV. Fifteenth ontogenetic stage: Brontops brachyce- 



phalus, Nat. Mus. 4947. 

 XIV. Fourteenth ontogenetic stage: Allops serotinus, 



Am. Mus. 520. 

 XIII. Thirteenth ontogenetic stage: Menodus giganteus, 



Am. Mus. 505. 



lower geologic level is prophetic of its absence on a 

 higher geologic level. This variability is especially 

 displayed in retrogressive structures such as the 

 degenerate incisor teeth in the Brontops series, as is 

 shown by the following formulas: 



Juvenile incisors, I{i^ . 



Adult incisors, I|^}- . 



The incisive teeth tend to drop out in the adults, as 

 observed in the type of Diploclonus tyleri Lull. 



Thus among the age characters are the foUowrng: 

 (1) Increasing size and rugosity of the skull, arches, 

 horns, and nasals; (2) distal expansion and rugosity of 



