796 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



the anteroposterior measurements in each of the grind- 

 ing teeth, p'-m^. 



4. The correlation of the proportions of the indi- 

 vidual tooth respectively with the proportions of the 



molars as compared with the contemporary horses, in 

 which the premolars are rapidly molarized; (4) rare 

 development of mesostyle (median ridge of outer wall) 

 of premolar; (5) feeble development of entoconid of 



Figure 716. — "Brain easts" (intracranial casts) of titanotheres 



A, Palaeosyops leidyi, Am. Mus. 1544, left side view; B, Mesatirhinus pctersoni, Princeton Mus. 

 10041, left side view; C, MesatirhiTius -peicrsoni, Am. Mus. 1509, right side view; Di, Menodus 

 giganteus i=" BrontotheTtum ingejis" of Marsh), left side view, from a cast made under the direction 

 of Professor Marsh; D2, the same, top view. All one-third natural size. In Palaeosyops (A) we see 

 the very large olfactory lobes and the small cerebrum. The brain of Mesatirhinus (B, C) appears 

 to be shorter anteroposteriorly. 



brachycephalic or dolichocephalic skull is well shown 

 in the curves (fig. 719), the contrast between the doli- 

 chocephalic and the brachycephalic forms of grinding 

 teeth being very clear, as well as the progressive 

 broadening of the grinders with the progressive bra- 

 chycephaly of the Brontops hrachycephdlus and the 



B. rohustus phylum. 



5. The Brontops and Menodus curves of the short- 

 horned titanotheres are of the same general type, con- 

 trasting with those of the Brontotheriinae. 



6. In the Brontotheriinae p*-m' are typically broad; 

 in Menodus they are typically narrow. 



Although more numerous measurements are needed 

 to make these results absolutely precise or decisive, 

 the general conclusion is sustained that each tooth in 

 each genus and species has its distinctive average and 

 typical proportions. 



ARRESTED EVOIUTION IN THE TEETH 



The teeth of the titanotheres show the following 

 instances of arrested or retrogressive evolution: (1) 

 Ketrogressive evolution of the incisors; (2) retro- 

 gressive evolution of the canines in many series; 

 (3) imperfect or retarded molarization of the pre- 



FiGUBE 717. — Evolution of the upper molar 

 tooth in titanotheres 



Posterior view of a series of left upper molars, showing the gradual 

 deepening of the ectoloph, the persistently low, conic form of 

 the protocone and hypocone, and the loss of the conules. A, 

 Eoiiianops borealis, lower Eocene; B, Limnohyops priscus, 

 middle Eocene; C, Manleoceras washakiemis upper middle 

 Eocene; T>, TeJmaihenum uUimum , upper Eocene; E, Menodus 

 giganteus, lower Oligocene. 



the premolars; (6) hypsodontism of the molars con- 

 fined to the ectoloph, or outer wall of the crown, a 

 mechanically imperfect evolution; (7) elongation of 

 the molar crowns prevented by the position of the 

 orbits immediately above the grinding teeth. 



