394 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



metastyles; (5) the reduced but still persistent proto- 

 conules (sometimes vestigial, Am. Mus. 1519, 1513, 

 M. megarhinus); (6) the anterior and posterior cingula; 



FiGTjRE 331. — Lower jaws of Mesalirhinus 



One-tourth natural size. A, if. megarhinus, Am. Mus. 1520, Bridger Basin, Wyo., 

 level unknown, B, M. petersoni, Am. Mus. 1512, La Clede Meadows, Washakie 

 Basin, Washakie A; symphyseal region restored from Am. Mus. 1575 (.M. mega- 

 rhinus), La Clede, Washakie Basin; canine from Am. Mus. 1551 (M. megarhinus). 

 Twin Buttes, Bridger Basin. C, M. petersoni, Am. Mus. 1667, Bridger Basin, 

 level unknown; lower jaw of an old animal. D, M. petersoni, Am. Mus. 13178, 

 north of Haystack Mountain, Washakie Basin, Washakie A; lower jaw of an 

 old animal. 



(7) the vestigial metaconule seen in m^ only in certain 

 specimens (Am. Mus. 1556), the majority exhibiting 

 no trace of this cusp; (8) the serrate external cingulum 

 at the bottom of the ectoloph valleys, especially in the 

 more progressive specimens. 



The superior molar series measures from 87 to 91 

 mUluneters in M. petersoni and from 77 to 83 in M. 

 megarJiinus. The inferior molar series measures from 

 94 millimeters in M. megarhinus to 104 in M. petersoni. 



The inferior molars are characterized by faint 

 serrate, noncontinuous external cingula, which follow 

 the curvature of the crown inward between the outer 

 lobes, as distinguished from the cingula in P. paludosus, 

 which form a straight line along the base of the outer 

 border of the tooth. In the long, narrow ma {M. 

 megarlhinus, ap. 43 mm., tr. 19; M. petersoni, ap. 46, 

 tr. 19) a serrate internal cingulum rises on the inner 

 side of the hypoconulid but does not ascend so 

 prominently as in T. cultridens. This hypoconulid is 

 progressively conic in form; it is slightly more conic, 

 more median in position, and less sharply crescentic or 

 cupped on the inner side than in T. cultridens; but in 

 certain specimens (Am. Mus. 1512, 1577) it has the 

 moie crescentic form of the T. cultridens type. In 

 some molars (Am. Mus. 1512, 1575, 1520) faint rudi- 

 ments of the metastylid fold are seen, but as a rule the 

 internal valleys are open and smooth. Other teeth are 

 too much worn to determine the presence or absence of 

 the metastylid fold. 



Lower jaws of M. megarJiinus and M. petersoni. — 

 The lower jaw of these animals is represented by a 

 large number of separate jaws belonging to both 

 species (see below). These jaws taken together afford 

 very complete knowledge of the progressive, age, and 

 sexual characters. There is a very marked disparity 

 in size between the smallest (Am. Mus. 1520, M. 

 megarhinus) and the largest (Am. Mus. 1512, M. 

 petersoni). 



Comparative measurements of Mesatirhinus and Metarhinus, in 

 millimeters 



Pi-ms 



Pj-ms 



Mi-m3 



Ms, anteroposterior 



Incisive border to angle. 



Mesatirhinus 

 megarhinus, 

 Am. Mus. 1520 

 (Bridger D?) 



162 



146 



94 



43 



Mesatirhinus 



petersoni. 

 Am. Mus. 1512 

 (Washakie A) 



176 

 ■160 

 102 

 46 

 325 



Metarhinus 



fluviatilis. 



Am. Mus. 1946 



(Uinta B 2) 



161 

 157 



102 

 46 



The coronoid rises rather rapidly behind m3, with 

 a more or less rounded or angulate anterior border and 

 with nearly parallel anterior and posterior contours 

 until near its summit, when it suddenly curves back 

 into a decided posterior hook. (Am. Mus. 1512, 

 fig. 331.) 



