EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



511 



the infraorbital foramen being very broad, even 

 broader than in Menodus giganfeus; there is a pre- 

 orbital knob on the lacrimal, which is seen also in 

 M. trigonoceras . The zygomatic arches, as shown in 

 all three views of the skull, are deep and narrow, not 

 expanding widely, and resembling those of the Menodus 

 type. There is a narrow contact between the post- 

 tympanic and postglenoid processes, and the par- 

 occipital process is narrow in palatal view (PL CXII), 

 wherein the dolichocephalic structure is again ap- 

 parent but is somewhat exaggerated in this type by 

 lateral crushing. 



Dentition. — There are two incisor alveoli which show 

 that these teeth had not undergone the degeneration 

 observed in members of the Menodus phylum. The 

 right canine so far as preserved shows somewhat less 

 anteroposterior compression than in the type of A. 

 marshi. The premolars of A. walcotti exhibit pro- 

 nounced internal and faint external cingula and feebly 

 developed tetartocones (fig. 430). The tetartocones, 

 however, are better developed than in the contem- 

 porary B. hrachycephalus , since the constriction sepa- 

 rating the tetartocone from the deuterocone in p-~^ is 

 a little more anterior in position. Furthermore, the 

 tetartocone of p* is not a concave spur from the 

 cingulum as in B. h'acJiycepJialus 

 or B. dispar. Characteristic 

 features are the crenulation of the 

 internal face of the deuterocones 

 and the broad internal cingula. 

 In regard to the proportions of 

 the grinding teeth, comparison 

 with the average anteroposterior 

 and transverse diameters of the 

 grinding teeth of five skulls of B. 

 iracJiycepJialus shows that in this 

 specimen the teeth are less mark- 

 edly brachyodont, a fact in accord- 

 ance with its supposed affiliations 

 with the Allops phylum. The 

 teeth are, in fact, as elongate as in 

 members of the Menodus phylum, 

 but this may be due partly to the 

 lateral crushing. Detailed meas- "' 



urements of the tooth proportions indicate, however, 

 that they are closer to B. hracJiyce2)halus than to those 

 of M. trigonoceras . 



Characters oj lower jaw. — A lower jaw (Nat. Mus. 

 4247, fig. 413) agi'ees exactly in size and may be pro- 

 visionally associated with this species. It presents 

 the following characters: The coronoid is elevated; 

 the angle projects downward and backward; the inen- 

 tal foramen is below ps. ly, P^. The canines are 

 pointed, erect, with an incomplete internal cingulum 

 and somewhat flat inner face; pi is entirely wanting, 

 a variable character; the cingulum on the remaining 

 grinders is moderately developed; nis exhibits an in- 

 cipient crenulate internal crest of hypoconulid. 



Measurements of jaw of Allops walcotti, Nat. Mus. 4B47 



Millimeters 



Angle to symphysis .525 



Condyle to angle 235 



Canines, anterior enamel 34 



Canines, anteroposterior 21 



Ms, anteroposterior 88 



M3, transverse 38 



Pa-mj 290 



This lower jaw has long, conical canines; it should 

 be compared with the lower jaw of Am. Mus. 1495. 



Allops marshi (Osborn) 



(Megacerops marshi Osborn, 1902) 



Plates XXXVIII, CXII-CXVI; text figures 197, 378, 381, 389, 

 391, 397, 399, 409, 429, 431, 432, 615, 712 



[Foi- oi-iginal description iiud type refersDces see p. 233. For slceletal characters 

 see p. 678] 



Geologic horizon. — The geologic levels of the type 

 and paratype of Allops marshi in the American 



Figure 430. — Upper teeth of Allops walcotti 

 . Mus. 8753 (paratype); m^ is missing. One-half natural size. 



Museum are not recorded, but they are probably from 

 upper A or lower B. A fine skull (Field Mus. P 6900) 

 is recorded as from .50 feet above the Pierre shale and 

 50 feet below the level of a skull of Brontotherium 

 hatcheri — that is, probably the upper levels of A or the 

 lower levels of B. A female skull (Nat. Mus. 1215) 

 slightly smaller than A. marshi is recoi'ded from 

 middle B. 



Specific characters. — Skull longer than in Brontops 

 hrachycephalus (645 to 675 mm., average 665), but 

 shorter than in B. dispar, of mesaticephalic proportions ; 

 zygomatic index, 64 to 69, average 66, and thus similar 

 to females of B. hrachycephalus. Horns (105 to 140 

 mm.) longer than in B. hrachycephalus ; basal section 



