522 



TITANOTHERES OP ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



From this it will be seen that in our present collec- 

 tions there are considerable gaps between the suc- 

 cessive stages so far as regards the measurements 

 p'-m^, p'-p^, and m'-m', but that the other measure- 

 ments overlap. The nasals in this phylum remain 

 very long, whereas in other phyla they are reduced in 

 length. 



This is a remarkably consecutive and distinct 

 phylum. Additions to the already large number of 

 skulls may give a continuous series of measurements 

 in each column. 



Geologic level. — Unfortunately, the records of the 

 geologic level of members of this phylum are by no 

 means so exact as those of members of other phyla. 

 No member of this phylum is certainly recorded from 

 the lower Titanotherium zone of South Dakota, 

 although the primitive characters of the oldest known 

 species, M. heloceras, indicate that it belongs on this 

 lower level. In Wyoming one skull of M. Jieloceras 

 (Am. Mus. 14576) has been recorded from the base 

 of the Titanotherium zone. Only one skull (Nat. 

 Mus. 4257), belonging to the species M. trigonoceras, 

 is definitely recorded as from the middle TitanotTierium 

 zone (level B). Another skull, referred to M. proutii 

 (Nat. Mus. 4701), is tabulated from the lower level 

 of C. This uncertainty ceases, however, in the upper 

 beds, in which we have two positive records — namely, 

 Nat. Mus. 4291, entered as from the upper Titano- 

 tTierium zone (C), and a skull in the Field Museum 

 (P 5927), recorded as found near the top of the upper 

 Titanotherium zone, or upper C. It is thus probable 

 that Menodus persisted to the very end of the great 

 titanothere epoch. 



SYSTEMATIC DESCRIPTIONS OF GENERA AND SPECIES IN 

 THE MENODUS PHYLUM 



Menodus Pomel, 1849 



(Tiianoiherium Leidy , 1852; Symborodon Cope (<S. tonus, jaw 

 only); "Titanotherium Leidy," Osborn, 1902) 



Plates XVIII, XX, XXII, XXIV, XLVII, XLIX, LXXXII, 

 CXXIII-CXLII, CLVII-CLIX; text figures 24, 159, 160, 

 166, 168, 172, 175, 181, 227, 228, 375, 378, 381, 382, 387-389, 

 391, 393, 394, 396, 398-400, 406, 409, 435-447, 613-619, 630, 

 639, 640, 642, 701, 708, 713, 715-719, 744, 746 



1 For original description and type references see p. 204. For skeletal characters see 

 p. 6811 



Generic characters. — Characters 3-9, 14, 21, 23, 24, 

 26 (see below) are expressions of the general tendency 

 to dolichocephaly. 



Dentition. — (1) I|^. Incisors vestigial. (2) Ca- 

 nines, (? large, elongate (42-70 mm.), with slight 

 anterior and strong postero-internal cingulum; pos- 

 terior face rounded; 9 slender, small. (3) Opposite 

 grinding series rectilinear (not arched). (4) Upward 

 flexure of premolar series, as seen in side view, slight. 

 (5) Length of premolar-molar series about equal to 

 one-half that of the skull, from premaxillary tips to 

 occipital condyles (dental index 48 to 51). (6) Pre- 

 molar series long. (7) Internal cusps of grinding 

 teeth with sides progressively steep, ectolophs more 



nearly vertical than in Brontops, external crescents of 

 molars more open. (8) Anteroposterior- diameter of 

 m^, m' usually greater than transverse diameter. (9) 

 Cingula present between grinders. (10) P|;f; p^ 

 with worn crown rounded or pear-shaped in outline, 

 outer wall overlapped posteriorly by ectoloph of p^. 

 (11) Premolar tetartocones exhibiting rapid progres- 

 sive development. (12) Premolars with pronounced 

 internal cingula, edge rounded to sharp, external 

 cingula sharp, well marked. (13) Molars with partial 

 internal cingula pronounced, external cingula pro- 

 nounced. (14) Hypocone of m' often separate, and 

 surrounded by cingulum. 



Skull. — (15) Skull proportions dolichocephalic. (16) 

 Facial portion of skull elongate, with premaxillaries 

 not reduced. (17) Cranial part of skull very elon- 

 gate. (18) Preorbital malar bridge broad to very 

 broad, with median (malar) ridge low and obtuse 

 {M. heloceras, M. trigonoceras) or weU rounded {M. 

 giganteus) . (19) Infraorbital foramen very conspicuous 

 in side view. (20) Malar below postorbital process 

 subflat, deep. (21) Free portion of nasals persistently 

 elongate, with parallel sides; but sometimes expanding 

 distally in old individuals. (22) Horns of small to 

 medium size, originating over preorbital malar ridge, 

 shifting forward progressively;^* basal section pro- 

 gressively trihedral, obliquely placed; summit of horn 

 round to trihedral. (23) Zygomata slightly arched; 

 buccal expansions slight or moderate, in section deep 

 rather than broad. (24) Occiput moderately pro- 

 duced backward behind zygomata; vertex of skull 

 broad posteriorly; pillars flaring slightly to moder- 

 ately; occiput indented; with median knobs slight or 

 absent. (25) Basisphenoidal rugosity usually present; 

 vomerine septum present. (26) Jaw deep, elongate 

 ramus, pronounced chin; coronoid uniformly broad, 

 elevated, square at summit; angle depressed or back- 

 ward extended. 



The contrasts in proportions of the terminal mem- 

 bers of the Brontops, Menodus, and Brontotherium 

 phyla are shown in the following table: 



Measurements of typical forms of Brontops, Brontotherium, and 

 Menodus, in millimeters 



Pmx to condyles 



Zygomata, transverse, 



P>-m3 



Dental index 



Pi-p« 



M'-m3 



P', ap. by tr 



M3, ap. by tr 



Brachy- 

 cepbalic: 

 Brontops 

 robustus, 



Yale Mus. 



12048 (type) 



766 



667 



350 



46 



137 



220 



40X65 



81X92 



Dolicbo- 



cephabc: 



M. giganteus, 



Field Mus. 



P6927 



(typical) 



Hyperbracby- 



cepbalic: 



Brontotherium 



gigas elatum, 



Am. Mus. 492 



(typical) 



825 



515 



425 



51 



150 



270 



- 58X73 



"■100X78 



830 



740 



353 



42 



126 



241 



47X72 



91X99 



1 Univ. Wyoming Mus. 



" A surprising fact is that the horns in the unerushed M. gigantetis (Field Mus. 

 P 5927) are almost as far forward as in the unerushed B. gigas. In the unerushed 

 B. platyceras and B. ramosum the forward displacement appears extreme. 



