518 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



referred to the subgenus AUops in the following 

 points : 



1. Greater skull length, 750 millimeters; others 640 to 730. 



2. Greater length of grinding series, 370; others 310 to 335. 



3. Greater length of premolars, 155; others 131 to 140. 



4. Greater length of molars, 220; others 192 to 215. 



5. Greater length of canine; anterior face 45; others 33 



to 40. 



This uniform increase in size in all measurements 

 implies that A. crassicornis comes from a higher 

 geologic level than any of the other forms and is in 

 thorough accord with the advanced condition of the 

 tetartocones. 



Geologic and geographic distribution. — The geologic 

 level recorded for this specimen by its discoverer, 

 Hatcher, is the upper part of the middle beds; in other 

 words, it is the same as that assigned to the type and 

 the female of A. serotinus. A marked progression in 

 the evolution of the premolars is, however, posi- 

 tive proof that this animal belongs to a very much 

 higher geologic level than A. serotinus, though there 

 is little doubt as to its phyletic descent from that 

 species. 



Materials. — Four skulls m the National Museum 

 (Nos. 4289 (type), 2117, 4709, 8740) are referred to 

 this species. 



Description oj type. — The type skull (Nat. Mus. 

 4289) is undoubtedly that of an old male. The nasals 

 are square and heavy, considerably longer than in 

 Brontops rohustus but much shorter than in Menodus 

 giganteus. The horns are set very wide apart, and 

 it is a noteworthy fact that they entirely lack the con- 

 necting crest which is so distinctly developed in speci- 

 mens of Menodus. The basal section reveals the 

 affinity to A. serotinus; it is distinctly trihedral, and the 

 long axis is oblique rather than transverse as in Bron- 

 tops rohustus. In proportions this skull is dolicho- 

 cephalic; ratio of length(750 mm.) and width (570 mm.) 

 gives a zygomatic index of 75; the measurements are 

 rendered less exact, however, by the artificial spreading 

 of the left side of the type skull. The proportions are 

 intermediate between the mesaticephalic and the 

 dolichocephalic types. 



Dentition. — We are first impressed by the reduced 

 size of the superior lateral incisors (PI. CXXI), which 

 are more vestigial than in any member of the true 

 Brontops series. The canines are a very distinctive 

 progressive development of the A. serotinus form, 

 attaining a length of 45 millimeters. The fourth 

 supei'ior premolar has a low but sharp and distinct 

 tetartocone. The second and thii'd pi'emolars also 

 have well-defined tetartocones. The third superior 

 molar has a peculiar sharp elevation of the cingulum 

 but no hypocone. The premolars and molars either 

 parallel or indicate their affinity to those of Menodus 

 in the decided development of the external cingula; 

 P^i P') P^ also exhibit strong external cingula. 



THE MENODUS MONOPHYLUM 



(" Titanotherium phylum," Osborn, 1902; " Symborodon" torvus 

 (jaw), Menodus, Menops, Diconodon) 



Like Brontotherium, the genus Menodus includes a 

 nearly continuous series of ascending mutations from 

 the base to the summit of the Titanotherium zone, 

 which may be considered nearly if not quite m'ono- 

 phyletic. The lines of separation between so-called 

 "species" are wholly arbitrary. 



Briefly stated, the distinctions of the Menodus 

 phylum as a whole are as follows: (1) Skulls dolicho- 

 cephalic, indices 62-70; (2) premolars rapidly pro- 

 gressive in molarization; (3) superior incisors aborted; 



(4) horns abbreviated, triangular, not shifted forward; 



(5) high dental index; (6) canines large, piercing. 

 Phyletic characters. — Large, long-limbed or cursorial 



titanotheres, long skulled and short horned. Ranging 

 from the lower to the upper Titanotherium zone. 

 Vestigial incisor teeth and long, pointed canines. 

 Grinding series elongate, with a high dental index. 

 Crowns of grinders subhypsodont and with promi- 

 nent cingula. 



These menodonts constitute one of the most sharply 

 defined of all the titanothere phyla. The elongation 

 of the feet and of the limbs indicates that of all the 

 titanotheres Ivnown they were the most rapid travelers. 

 While sparsely known in the lower Titanotherium 

 zone and somewhat more abundant in the middle 

 Titanotherium zone, they were relatively numerous 

 in the upper zone, an indication of the more favorable 

 conditions which this region presented toward the 

 end of the Titanotherium period for animals of this 

 type. They appear to have divided the honors with 

 the great long-horned brontotheres, although their 

 i-emains are much more rare. 



The short, stout, pointed horns stood in wide con- 

 trast, however, with the great recurved horns of the 

 brontotheres. While the canine tusks may have 

 served as minor weapons of defense, it appears probable 

 that the menodonts, as the swiftest members of this 

 great group, had recourse to flight when attacked by 

 herds of carnivores. 



To the anatomist the menodonts are extraordinarily 

 interesting in presenting extremes both of dolicho- 

 cephaly and of dolichopody — that is, of long-headed, 

 long-limbed, and long-footed development, in contrast 

 with the broad headed. The group affinities of these 

 animals with members of the Brontops phylum are in- 

 dicated by the less intermediate characters of the 

 Allops phylum. Whereas all other titanotheres ex- 

 hibit progressive abbreviation of the nasal bones, in 

 these animals the nasals retain the broad, elongate 

 form first seen in the upper Eocene Protitanotherium 

 emarginatum. 



As compared with the more or less intermediate 

 genus Allops these true titanotheres are more dolicho- 



