EVOLUTION OF THE SKTJLL AND DENTITION OF OLIGOCENE TITANOTHERES 



509 



The Allops phylum as represented in the Hatcher collection of 26 skulls and lower jaws from the Chadron formation 

 in the United States National Museum — Continued 



Allops walcottl Osborn 



Plates XX, XXI, CXI, CXII: text figures 207, 389, 391, 394, 



397, 409, 413, 429, 430 



[For original description and type references see p. 241) 



Geologic horizon. — Lower Titanotherium zone (Chad- 

 ron A) of South Dakota. 



Specific characters. — Premolars with small tetarto- 

 cones; p'-m' 285 millimeters. Incisors %. Horns 

 elongate oval, no connecting crest. Mesaticephalic. 

 Nasals elongate, broad. Face relatively elongate. 



The type skull of this species (Nat. Mus. 4260) from 

 Chadron A is narrow and elongate, partly owing to 

 lateral crushing. This feature conceals its resem- 

 blance to Allops marshi, which is apparent in other 

 features — namely, (1) primitive, long nasals, (2) horns 

 primitively short and obliquely oval, (3) large lateral 

 incisor (i2) and small first (ii) or median incisor, 

 (4) premolars accelerated, tetartocones more advanced 

 than in Brontops rohustus of level C. 



Observations on the measurements of Allops walcotti. — 

 The type of this species exhibits the following measure- 

 ments in comparison with skulls of B. hrachycephalus 

 and Menodus heloceras, which shows that the type of 

 Allops walcotti has relatively large premolars and small 

 molars. 



Measurements of Allops walcotti, Menodus heloceras, and Bron- 

 tops hrachycephalus, in millimeters 



Pi-m3 



Pi-p< 



M"-m3 



Pmx to condyles 



Nasal length 



Horn length 



PS ap.Xtr 



MS ap.Xtr 



A., walcotti, 

 Nat. Mus. 

 4260 (type) 



285 

 112 

 169 

 640 

 105 

 100 

 35X51 

 60X61 



M. heloce- 

 ras, Am. 

 Mus. 14576 



265 



170 



603 



132 



70 



B. brachycephalus 



Nat. Mus. 

 4940, 9 



265 

 101 

 160 



102 

 32X51 

 62X70 



Nat. Mus. 

 4261, cT 

 (type) 



280 



»104 



178 



580 



85 

 33X53 

 68X73 



' Estimated. 



The skull is crushed laterally, but probably had a 

 low zygomatic index — that is, it was mesaticephalic. 

 While its reference to Allops requires confirmation, its 

 nearer affinities appear to be with this genus rather 

 than with Brontops or Menodus. The external cin- 

 gula of the premolars are not as sharply defined as in 

 other primitive members of the menodontine group. 



Geologic and geographic distribution. — This species 

 is represented at present by a single skull, the type 

 (Nat. Mus. 4260), which is recorded as probably from 

 the lower levels (A) of the lower Titanotherium zone 

 of South Dakota. 



Comparison with other species. — This animal should 

 naturally be compared with other titanotheres from the 

 lower beds. It is readUy distinguished from B. 

 brachycephalus by a number of characters as follows : (1 ) 

 The skull is much more doHchocephalic, a feature that 

 is intensified by lateral crushing (see below); (2) the 

 nasals are long and subquadrate in form; (3) the 

 individual measurements of the grinding teeth show 

 that the series throughout is somewhat narrower than 

 the grinding series in B. brachycephalus. 



These contrasts with the broad-skulled B. brachy- 

 cephalus naturally suggest comparison of this animal 

 with primitive members of the long-skulled Menodus 

 phylum, such as M. heloceras, remains of which from 

 the lower Titanotherium zone are sparsely known. 

 Comparison with M. heloceras shows that A. walcotti 

 possesses the following distinctions: (1) Horns more 

 elongate-oval in section, less trihedral; (2) no con- 

 necting crest between the horns; (3) alveoli for two 

 incisor teeth of considerable size (incisors are extremely 

 vestigial in the Menodus phylum). 



There remains the comparison with Allops marshi, 

 the form to which, on the whole, this cranium seems 

 to present the largest number of resemblances. The 

 animal differs from A. marshi in the inferior dimensions 

 of the dental series as well as in the detailed propor- 

 tions of the teeth and the characters of the premolars, 

 but it appears to be an older and more primitive form 

 which belongs to the A. marshi phylum. In _ the 

 horn section, in the proportions of the nasals, in the 



