514 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



The British Museum specimen (No. 5743 M) 

 above described agrees well with the type and para- 

 type in all measurements except in the zygomatic 

 index, which is low and agrees with that of a supposed 

 female (Nat. Mus. 1213) from the lower beds. 

 The horns are more erect than in the paratype- 

 The tetartocone development is about the same as in 

 the type. 



The Field Museum specimen (No. P 6900) agrees 

 well with the type in measurements but differs in the 

 somewhat more backwardly prolonged occiput; the 

 canines, which are large, indicate that this specimen 

 is probably a male. It is more fully described below. 



Figure 432. — Lower jaws of Allops marshi and Allops? sp. 



A, Allops marshi, Field Mus. P 6900; ramus sweeping gently forward with lower border 

 nearly horizontal and angle not produced downward, chin slightly convex, canine appressed 

 to P2, external cingula not sharply defined. B, AUops'i sp., Ottawa Mus.; Cypress Hills, 

 Saskatchewan. This specimen (one of Cope's cotypes of M, ang-ustigenis) differs from the 

 typical Menodus in the presence of incisors (as indicated by the alveoli) and in the somewhat 

 less hypsodont form cf the grinding teeth; the angle is not produced backward. The refer- 

 ence to Allops is provisional. About one-si.\th natural size. 



The skull in the National Museum (No. 1213) 

 agrees closely with the type of A. marshi in measure- 

 ments and is somewhat more progressive in premolar 

 evolution than the type of B. hrachyceplialus. 



Another skull (Nat. Mus. 1215) is recorded from 

 the middle levels of B and may pertain to this species, 

 although the occiput is more prolonged backward than 

 in the type; the premolar tetartocones might represent 

 either this species or B. dispar. 



Equally interesting is the skull Nat. Mus. 1214, 

 referred to B. hrachycej>7iahis but possibly an ancestor 

 oi A. marshi. 



Detailed characters of Allops 7narshi. ^-The. detailed 

 characters of the type (Am. Mus. 501) are as follows: 

 The skull, apparently belonging to a female, is in the 

 seventh growth stage. The incisive border exhibits 

 alveoli for a large lateral and small median incisor. 

 The canines have short, obtuse, lanceolate crowns, a 

 posterior and a slight lateral cingulum. The first su- 

 perior premolar is bifanged, close behind the canine; 

 it exhibits no tetartocone. The tetartocones are less 

 advanced than in the contemporary 31. trigonoceras, 

 moderately developed on p' and p^ and forming a de- 

 pressed loop on p*; the external and internal cingula are 

 clearly defined. The molars exhibit partial cingula on 

 the ectoloph and rudimentary cingula on the 

 inner side. The hypocone of m'^ is connected 

 with the cingulum (in the paratype, however, 

 the hypocone is separated from the cingulum as 

 a low cusp) ; there is a rudimentary metaloph, 

 especially on m^. The postorbital process of the 

 frontal crest forms a rudimentary sharp angle; 

 the orbit is large; the malar bridge is broad, 

 with a low ridge; the buccal expansions of the 

 zygomata are slight in top view; the edges of 

 the frontal crest are seen to diverge widely, 

 running to the malar ridges of the horns. The 

 nasals are of medium length, deeply convex 

 above and concave below, projecting beyond 

 the premaxillaries anteriorly. 



The paratype skull (Am. Mus. 1445), prob- 

 ably that of a male, is, as above noted, more 

 progressive. In p* the tetartocone is more 

 distinctly budded off. As in the type the 

 canines have the short, sublanceolate section 

 and posterior lateral cingulum quite distinct from 

 the more pointed canines of Brontops dispar. 

 The Field Museum skull (No. P 6900) as- 

 sociated with a lower jaw and a manus of 

 Allops marshi, from Phinney Springs, S. Dak., 

 has been skillfully reconstructed by Mr. Riggs 

 and is now one of the least distorted titano- 

 there skulls of all that are known. It agrees 

 closely in measurements with the other skulls 

 referred to^. marshi and shows corresponding 

 specific characters. 



Detailed measurements of the upper grinding 



teeth prove that the transverse measurements 



exceed the anteroposterior measurements throughout. 



This proportionate excess of transverse diameter is 



progressive in this brachycephalic line. 



Ascending mutations. — This species, therefore, in- 

 cludes specimens which represent several "ascending 

 mutations" in increase of size, in the development of 

 the premolar teeth, in separation of the tetartocones, 

 and in expansion of the zygomatic arches. Of these 

 the geologically oldest perhaps is the National 

 Museum specimen No. 1214, which is distinctively a 

 B. hrachycephalus in its measui'ements. Next in order 

 of evolution comes the female specimen (Nat. Mus. 

 1213), distinctly an A. marshi in its measurements. 



