512 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



trihedral, more or less pointed or trihedral at the 

 summits. Nasals elongate, free length 98 to 105 milli- 

 meters. Incisors 2-1, large i^, small i^ Grinding 

 series, length 310 to 335 millimeters, average 319; 

 molars average 192; dental index same as in B. hrachy- 

 cepJialus and B. dispar, namely, 47. Canines strongly 

 compressed anteroposteriorly, length of crown 34 to 

 37 millimeters. Premolars with tetartocones of p^ 

 better developed than in B. hrachycepJialus, tetarto- 

 cones of p^ and p^ better developed than in B. 

 IracJiycepTialus and similar to progressive members of 

 B. dispar. Occiput not greatly prolonged back of 

 zygomatic arches. 



Observations on the measurements oj Allops marshi. — 

 From Brontops dispar the skulls of A. marsJii are dis- 

 tinguished above all by their very low zygomatic 

 index, 64 to 69, as compared with 77 to 87 in B. dispar. 

 This marked narrowness, together with the small size 

 of the canines (vertical diameter 34 mm., as compared 

 with 40 in B. (validus) dispar), has led to the suspicion 

 that Allops vfiarsTii may be the female of B. dispar. A 

 comparison of the paratype of Allops marsTii with the 

 type of B. validus is given below: 



Measurements of Brontops (validus) dispar and Allops marshi, in 

 millimeters 



P'-mS 



Pi-p< 



Mi-m3 



Canines : 



Vertical 



Anteroposterior 



Pnix to condyles 



Zygomatic index 



B. (validus) 



dispar, Nat. 



Mus. 4290 



(type) 



. raarstii, 



Am. Mus. 1445 



(paratype) 



320 

 130 

 203 



40 



27 

 660 



85 



335 

 135 

 203 



37 



22 



675 



64 



The relative widths of p* and m' also appear not to 

 differ very clearly in the two forms: 



Anteroposterior and transverse measurements of p* and m^ in 

 Brontops (validus) dispar and Allops marshi, in millimeters 



A. marslii is decidedly smaller than A. serotinus and 



A. crassicornis, and apparently no known skulls bridge 

 over this gap. It is larger in all measurements than 



B. hrachyceplialus. 



Materials. — This species is represented by 15 or 

 more skulls, including the type (Am. Mus. 501, Pis. 

 CXIV, CXV), a well-preserved skull; the paratype 

 (Am. Mus. 1445, Pis. CXIV, CXV) ; a well-preserved 



skull (Brit. Mus. 4446 M, PL CXIV) whose geologic 

 level is not recorded; an exceptionally perfect skull 

 (Field Mus. P 6900), associated with the lower jaw and 

 parts of the skeleton, from the upper levels of A or 

 the lower levels of B; a fine skull in the Museum of 

 Comparative Zoology, level not recorded, originally 

 described by Scott and Osborn (1887.1, p. 158) as 

 " Menodus coloradensis"; a cast of an unknown skull 

 (Carnegie Mus. 289) ; a skull, recorded from upper A, 

 probably a female (Nat. Mus. 1213); a skull from B 

 (Nat. Mus. 1215). 



Comparison of Allops with members of the Brontops 

 phylum. — Are these specimens females of B. dispar? 

 There is some evidence that the specimens attributed 

 to A. marshi are female forms of B. dispar, consisting 

 chiefly of the following items: (1) The dental indices 

 are the same; (2) the tetartocone development is prac- 

 tically identical; (3) the cephalic index is mesaticepha- 

 lic, like that of the females of B. dispar; (4) the speci- 

 mens of A. marshi are found on lower levels of the 

 range of B. dispar. On the other hand, the evidence 

 against regarding A. marshi as the female form of B. 

 dispar is somewhat stronger, as follows: (1) The skulls 

 of A. marshi are generally recorded from lower geologic 

 levels; (2) the skulls in the Field and Harvard museums 

 have larger canines, indicating that they are males; 

 (3) the occiput of A. marshi is not greatly prolonged 

 behind the greatest width of the zygomatic arches, in 

 contrast with B. dispar, in which the occiput is greatly 

 produced posteriorly (see diagram); (4) the horns of 

 A. marshi are more triquetrous or transversely oval, 

 while the horns of B. dispar are rounded; (5) the nasals 

 of A. marshi are much more elongate. 



Although the specific and phyletic distinction of A. 

 marshi from B. dispar thus appears certain, the 

 "group" affinity of the animals is very strong. We 

 observe (1) the pit in the vertex of the cranium, as 

 in B. brachycephalus , B. dispar; (2) the features in 

 which A. marshi differs from B. dispar tend to relate 

 A. marshi more closely to the succeeding form B. 

 rohustus. 



Among the similarities between A. marshi and B. 

 roSttsius are the following : (1) The form of the canine, 

 which tends to obtuseness; (2) the abbreviation of the 

 occiput behind the zygoma; (3) the broad, rugose 

 summit of the occipital pillars; (4) the breadth and 

 squareness of the nasals; (5) the presence of two knobs 

 on either side of the median line of the occiput for 

 the recti capiti muscles. The horns in their prophetic 

 growth do not acquire the trihedral section seen in 

 the Menodus phylum but tend to become more oval. 

 Thus in the paratype of ^. marshi (Am. Mus. 1445) we 

 find an approach to the transversely expanded horns 

 of B. robustus. Among the more primitive characters 

 of the skull distingmshing A. marshi irom B. rohustus 

 are the smaller size and less robust structure in general, 

 the shortness and obliqueness of the nasal section of 

 the horns, the greater length and slenderness of the 



