EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



489 



index 46 to 49, average 47. Canines pointed and 

 more or less compressed anteroposteriorly. Pre- 

 molars more progressive; tetartocones p^"^, more 

 advanced than in B. bracliycephalus. Tetartocone 

 of p^ progressively developed from a concave spur 

 from the deuterocone, continuous with the cingulum, 

 to a rudimentary cusp. Backward prolongation of 

 the occiput behind the zygomata very pronounced. 



This significant species connects closely with the 

 most progressive stages of Brontops hrachycepJialus 

 and with certain skulls which may be referred to 

 Brontops rohustus, although the typical B. dispar 

 does not appear to be directly ancestral to the typical 

 B. rohustus. 



The range in size in skulls referred to Brontops 

 dispar is given above. 



From Allops marsJii, as well as from Brontops 

 hrachycepJialus , the skull of this species is distinguished 

 especially by its greater robustness (zygomatic index 

 78-87, as compared with 64-71), larger horns, and 

 shorter, thicker nasals, as indicated in the following 

 measurements: 



Measurements of Brontops dispar and Allops marshi, in 

 millimeters 



Pi-m3 



P'-p4 



Mi-m3 



Canine: 



Vertical 



Anteroposterior. . 



Pmx to condyles 



Zygomatic width 



Cephalic index 



Nasal length ■_ 



Nasal breadth 



Horns, external length 



! . dispar, Nat, 



Mus. 4290 



(type of 



B. validus) 



320 

 130 

 203 



40 



27 



660 



565 



85 



85 



120 



198 



A. marshi, 



Am. Mus. 501 



(type) 



310 

 122 

 191 



34 

 25 

 673 

 450 

 67 

 100 

 105 

 105 



From Menodus proutii we observe that B. dispar is 

 distinguished by its brachycephaly, by the greater 

 relative width of its premolars and molars, and by the 

 more retarded development of the premolar tetarto- 

 cones, especially in p*. 



From Allops serotinus and A. crassicornis we observe 

 that B. dispar is usually distinguished by its higher 

 zygomatic index and by the lesser width of the pre- 

 molars. 



The type of B. dispar, unfortunately a badly 

 crushed skull, has less robust horns and nasals than 

 the type of B. validus, but the general agreement in 

 measurements as well as in dental characters appears 

 to indicate that these two types belong either to the 

 same or to very nearly related species. 



Geographic and geologic distribution. — According to 

 Hatcher's record, the typical B. dispar skulls were 

 found mainly in B or the middle Titanotherium zone, 



but they extend also into the lower levels of the upper 

 beds, namely, lower C (Chadron C 1). The four 

 ascending skulls in the National Museum most ac- 

 curately recorded by Hatcher and Stanton in 1901 

 are from 55 to 71 feet above the Cretaceous Pierre 

 shale, as follows: 



Skull 4258, Brontops bracliycephalus (transitional), 71.45 feet 

 above the Pierre. 



SkuU 4259, B. brachycephalus (transitional), 55.6 feet above 

 the Pierre. 



Skull 4290, B. dispar (type of B. validus), 62 feet above the 

 Pierre. 



Skull 4703, B. dispar, 62 feet above the Pierre. 



Geologic variation. — Inconsistent with the general 

 observation that the more progressive forms are found 

 on higher levels is the fact that the specimen first 

 named (Nat. Mus. 4258) is a very primitive form 

 which is distinctly related to B. hracTiycephalus, as 

 noted above. The type of Diploclonus tyleri, although 

 very advanced, was also found at a very low level — 

 namely, 35 feet above the Pierre shale, in 200 feet of 

 beds belonging to the Titanotherium zone (Lull). 

 These inequalities are certainly due to the uneven 

 surfaces of the Pierre shale on which the White River 

 was deposited. 



Materials of B. dispar. — This species is represented 

 in Hatcher's collection for the Geological Survey by 

 twenty-one or more skulls and several jaws now in 

 the National Museum, including especially the follow- 

 ing: The juvenile transitional skull (No. 4259) from 

 lower B, or the middle beds, referred to B. brachyce- 

 phalus; the male type of B. dispar (Nat. Mus. 4941) 

 from middle B; four finely preserved male skulls — 

 namely. No. 4703 from middle B, No. 4290 from 

 middle B (this specimen is the type of Brontops 

 validus), and Nos. 4245, 4248 from B. The two skulls 

 last named are in the seventh and ninth stages of 

 growth. The female sex is represented by No. 4738 

 from lower B. There is also a complete skull and 

 ower jaw (Nat. Mus. 1217) from the upper zone 

 (level C). 



This magnificent collection of well-recorded material 

 in the National Museum enables us to determine 

 positively the range of Brontops dispar from lower 

 Chadron B to the base of Chadron C. In this long 

 geologic range there are marked progressive advances 

 in evolution. 



B. validus a synonym of B. dispar. — Careful com- 

 parison of the measurements and characters of the 

 type of B. dispar (Nat. Mus. 4941), from middle B, 

 with the type of B. validus (Nat. Mus. 4290), also from 

 middle B, together with the supplementary evidence 

 furnished by the collection of skulls in the National 

 Museum, shows that these nominal species are identi- 

 cal. They both represent the brachycephalic, short- 

 horned titanotheres of the middle levels. Although 

 the type of B. validus has on each side two upper 

 incisors and four premolars, a study of other materia 



