EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 



369 



In p^, a larger tooth, we observe a more constant 

 enlargement of the tritocone. The cingulum in all 

 these premolars (except Am. Mus. 1532, the most pro- 

 gressive specimen) fails to encircle completely the 

 inner sides of the obtuse deuterocones. Comparative 

 measurements of the premolars are given below. 



The inferior premolars are represented in Am. Mus. 

 1566, 1563, 2353. Of these. No. 1566 represents the 

 least progressive stage. The premolar measurements 

 here given relate to this specimen. Pi (ap. 12 mm., 

 tr. 9) is a simple, laterally compressed cone. P2 

 (21 by 11) has an elevated protocone, slightly hollowed 

 on its antero-internal border, with a rudimentary 

 cuspule (=paraconid of molars) and a low postero- 

 external cusp (=hypoconid of molars). In ps (18 by 

 12) the cusp analogovis to the metaconid in the molars 

 is beginning to be constricted off from the protoconid 

 and is defined by a prominent antero-internal con- 

 vexity, and the anterior (trigonid) and posterior 

 (talonid) crescents are beginning to be marked, but 

 the entoconid, as in all middle Eocene titanotheres, 

 is not yet developed. In p4 (20 by 14) we have a 

 submolariform tooth lacking only the cusps analogous 

 to the paraconid and entoconid but with a well- 

 developed cusp analogous to the hypoconid. 



Molars. — The subquadrate sujJerior molars, which 

 vary in longitudinal measurement from 100 to 118 

 millimeters (a wide range of variation), as well dis- 

 played in five individuals, exhibit (1) progressive ex- 

 ternal cingula with internal cingula in the valleys; (2) 

 protoconules vestigial or wanting; (3) a marked 

 angulation of the postero-internal border of m^ 

 accompanied by a prominent elevatipn of the cingulum 

 at this point, and in one specimen (Am. Mus. 1511) a 

 small, distinct hypocone, which, however, may 

 represent a metaconule. All the molars in the speci- 

 mens at hand are too much worn to give the height of 

 the ectoloph, which was undoubtedly elongate in 

 the unworn condition and which slopes strongly 

 inward. 



Comparative average measurements {ap. by tr.) of molars of 

 Manteoceras, Mesatirhinus, and Dolichorhinus, in millimeters 



The inferior molars, well preserved in Am. Mus. 

 1566, 2353, 1563, are uniform in character, measuring 

 from 111 to 118 millimeters, with an incomplete 

 external cingulum which fdls the valleys and in No. 

 1566 rises behind the talonid into a rudimentary 



entostylid, or reduplication of the entoconid. Beside 

 this reduplication we observe an incipient metastylid 

 or reduplication of the metaconid. The teeth rapidly 

 increase in length as we pass from mi to ma, the 

 measurements in No. 2353 being mi 28 millimeters, 

 m2 35, ma 53. The lower molars show a strong sub- 

 family resemblance to those of Mesatirhinus but are 

 distinguished by their somewhat larger size (mi_3 110 

 mm. in Am. Mus. 1566, as compared with 106 in the 

 type of M. petersoni) and especially by their greater 

 breadth (ms (tr.) 23 mm.; in M. petersoni 18). In 

 Manteoceras also the cutting V's, as seen in crown 

 view, form more acute angles than in Mesatirhinus. 

 These differences are still more emphasized by com- 

 parison with Dolichorhinus hyognathus, which has 

 long, narrow molars, wide-angled V's, and a relatively 

 small hypoconulid on ms. 



Jaws. — Many fine jaws belonging to this animal 

 have been collected by the American Museum ex- 

 peditions in the 

 Bridger and W a- 

 shakie Basins. Un- 

 fortunately only a 

 few are associated 

 with the skulls, 

 namely, Am. Mus. 

 1545, 1587, 12204, 

 2353. Another not 

 associated is Am. 

 Mus. 1566 (Bridger D). 



As shown in the 

 plates and figures, 

 the jaws, like the 

 skulls, teeth, and feet, 

 show certain charac- 

 ters prophetic of the 

 Oligocene titano- 

 theres. These char- 

 acters are always 

 most clearly dis- 

 played in the aged 

 forms and are (1) 

 the depressed lower °^^'S!S^: 

 border of the angle; 

 (2) the pit in the an- 

 terior border of the coronoid behind ms; (3) the 

 breadth and vertical elevation of the coronoid proc- 

 ess and sharp recurvature at the summit; (4) the 

 greater prominence and convexity of the chin. 



The coronoid process is more robust and less 

 strongly recurved than in Telmatherium cultridens; 

 it is decidedly more elevated and less falciform and 

 recurved than in Dolichorhinus hyognathus. It is thus 

 more of the type which we should expect to find as 

 ancestral to the coronoid processes of the Oligocene 

 titanotheres. 



Figure 309. — Incisors and canines of 

 Manteoceras manteoceras 



Am. Mus. 12683, Sage 

 Creek Spring, Bridger Basin, Wyo., level D; 

 supposed male. B, Am. Mus. 12678, Henrys 

 Fork Hill, Bridger Basin, level C 5; aged female. 



