424 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



Superior premolar-molar series: The grinding series, 

 although of diminutive dimensions (145 mm. in the 

 type, which includes p', and 137+ mm. in the longi-' 

 tudinally crushed cotype), are really more progressive 

 in character than those of M. megarhinus in the fol- 

 lowing respects: (1) P' has a subquadrate instead of 

 elongate subtriangular crown, the trito- and deutero- 

 cones being subequal; (2) m^ has an elevated postero- 

 internal cingulum, which forms a subfunctional hypo- 

 cone in the type; (3) the ectolophs of p^ p*, as well 

 as of the molars, are relatively more elevated. 



Premolars: The premolars of the type measure 

 (ap. by tr.), p^, 13 by 14 mm.; p^ 15 by 19; p*, 17 

 by 23. The internal cingula are well defined and 

 progressive, completely surrounding the deuterocone of 

 p^ and nearly surrounding the deuterocone of p*. 

 The external cingula of p^ p' are not well marked 

 across the protocone but are very pronounced opposite 

 the tritocone; in p* they are strongly developed oppo- 

 site both cusps, and the protocone "rib" was also 

 pronounced, all marks of relatively advanced speciali- 

 zation. 



The coronal pattern is exhibited much better in 

 Am. Mus. 1946, in which the premolar measurements 



Figure 357. — Right lower premolars (pi-pi) 



of Metarhinus fluviatilis 

 Crown view. Natural size. Am. Mus. 1946; White River, 

 Uinta Basin, Utah, Uinta B 1. 



(ap. by tr.) are, p', 15 by 17 millimeters; p^ 15 by 20; 

 p*, 17 by 23. 



The most significant features of the premolars in 

 this specimen (No. 1946) are as follows: (1) The 

 deuterocones are large and give a well filled out 

 subquadrate inner contour to p^ p*; (2) the tritocones 

 are very large and progressive, especially in p', p*) 

 and have subflat ectoloph faces except in p^, which 

 has a more convex tritocone ectoloph; (3) the proto- 

 cones (antero-external cusps) have large, sharply 

 defined external ribs; (4) no tetartocones are .yet 

 present; (5) the external cingula are very advanced, 

 rising into prominent parastyles, faintly continuous 

 across the protocone base in p% p*, and better de- 

 fined opposite the tritocone; the external cingulum of 

 p* at the base of the tritocone surrounds a sharp 

 protuberance, emphasizing the gentle protuberance 

 seen here in M. megarhinus; (6) the internal cingula 



are well defined but still incomplete opposite the 

 deuterocone, the posterior cingulum of p^, p* very 

 broad; (7) the internal cones of the premolars and 

 molars are relatively elevated, and they have very 

 thick enamel, these conditions causing the partly 

 worn tips to be sharply truncate. 



Allowing for differences in wear and for some differ- 

 ences in level, we conclude that, from the evidence of 

 the premolars. No. 1946 is related to or referable to 

 M. fluviatilis. 



The inferior premolars, measuring 59 millimeters in 

 Am. Mus. 1946, are also more advanced than those of 

 M. megarhinus. Pi is more advanced than in that 

 species; it is more elongate, has a larger posterobasal 

 swelling, a lower, rounder tip, and a faint rudiment of 

 the anterior valley. There is a faint external cingulum 

 on the anterior and posterior end. Its measurements 

 are 10 by 6 millimeters (ap. by tr.). P2 is also more 

 advanced than in Mesatirhinus, with a lower trigonid, 

 a somewhat higher talonid, and better-defined an- 

 terior and posterior valleys. Its measurements are 

 16 by 95 millimeters. In P2-Pi the protoconid forms 

 a high, blunt transverse ridge, extending internally 

 into the cusp analogous to the metaconid. In ps 

 (ap. 16 mm., tr. 10) the molariform tendency is still 

 more pronounced, the posterior lobe being stronger, 

 with rudiments of the internal styles appearing. 

 P4 (ap. 18 mm., tr. 13) exhibits a somewhat more ac- 

 cented external cingulum, and the cusp analogous to 

 the entoconid is much more elevated than in Mesa- 

 tirhinus and only less elevated than in the molars. 

 Molars: The lower molars do not differ greatly from 

 hose of M. petersoni, except that they are of slightly 

 smaller size (98 to 103 mm.), have the external cingula 

 a little better defined, are perhaps slightly more hypso- 

 dont, and have a more conic hypoconulid on ma. In 

 M. fluviatilis, as in M. megarhinus, the hypoconulid of 

 m3 is of small size and variable (or progressive) from 

 a crescentic to conic form. 



Lower jaws of Metarhinus fluviatilis; type and re- 

 ferred specimens. — The type skull lacks the jaw. The 

 superior grinding series (Am. Mus. 1946) is associated 

 with the jaws, and they are also found in the nearly 

 perfect jaw of Am. Mus. 2059 and in the left ramus of 

 Am. Mus. 1865, which is from Uinta B 1 . The diminu- 

 tive jaw forming Am. Mus. 2059 barely exceeds in 

 length the jaws of certain specimens of Eotitanops 

 horealis, but the rami are relatively deeper, and the 

 dentition is of course far larger and more advanced. 

 Measurements of the lower jaws of specimens of 

 Mesatirhinus and Metarhinus are given in the ac- 

 companying table: 



