EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHEEES 



517 



nasals are more elongate; (4) the zygomata are less 

 widely expanded; (5) the canines are of feebler dimen- 

 sions. In the grinding teeth the dental index, 50, 

 is remarkably high, but the actual linear measurements 

 of p'-m^, p'-p*, m'-m^ are the same as in the type 

 male skull. This is in accordance with the general 

 principle which we have found throughout titanotheres, 

 that the females while inferior in other characters 

 frequently present grinding teeth equal in size to 

 those of the males. 



Transitional skull or ascending mutations. — The 

 American Museum skull No. 520, as seen from above 

 (PL CXVII, B), resembles the type skull of A. sero- 

 tinus in many respects. As seen from below (PI. 

 CXVIII) it has the true subgeneric characters of 

 Allops, especially in the peculiar transverse, lanceolate 

 form of the canine teeth and in the roimded form of 

 the single incisor tooth. On the other hand, it differs 

 from the type of A. serotinus and appears to be transi- 

 tional toward a higher type in the decidedly greater 



Figure 434. — Coossified nasals and proximal part of 

 horns of Allops f serotinus? 



Specimen from Cypress Hills, Saskatchewan, Canada, in tlie Ottawa 

 Museum, referred by Lambe to Megacerops assiniboiensis, One- 

 tbird natural size. 



abbreviation of the nasals. In top view the horns 

 are seen to be intermediate between those of A. 

 serotinus and A. crassicornis. Similarly, while the 

 nasals are shorter than in A. serotinus they are nar- 

 rower than in A. crassicornis. The linear measure- 

 ments of the grinding teeth (330 mm.) agree precisely 

 with those of the male and female specimens of A.' 

 serotinus, and a still more conclusive resemblance is 

 seen in the decidedly retarded development of the 

 tetartocones, so far as one can judge from their worn 

 condition. 



Skull Nat. Mus. 4938. — This is another skull which 

 combines the primitive premolar structure of A. 

 serotinus with the more abbreviated nasals and more 

 massive form of A. crassicornis. The detailed meas- 

 urements of the teeth in this skull agree more closely 

 with those of A. serotinus than with those of A. crassi- 

 cornis. On the other hand, in both size and shape 

 of the sections of the horns the skull appears to agree 



more closely with A. crassicornis. Together with the 

 skull above described (Am. Mus. 520) this skull might 

 be cited to prove the existence of transitions between 

 the two successive stages. 



Summary. — The type (Nat. Mus. 4251) and the three 

 other skulls provisionally referred to A. serotinus — 

 namely, the female (Nat. Mus. 2151), the transitional 

 form (Am. Mus. 520), and the second transitional 

 form (Nat. Mus. 4938) — all agree in the retarded 

 state of evolution of the tetartocones, which are little 

 further advanced than in the species Brontops dispar. 

 The characters m which they disagree with B. dispar 

 are found in the horns and nasals, and these either 

 represent ascending mutations of the A. serotinus type 

 or more probably progressive stages toward the next 

 higher species, A. crassicornis. 



Allops crassicornis Marsh 



{"Allops crassicornis" Osborn, 1902) 



Plates CXIX-CXXII; text figures 189, 387, 409, 433, 608, 612 



[For original description and type references see p. 228. For skeletal characters 

 see p. 6791 



Geologic horizon. — Titanoiherium zone of South 

 Dakota, middle or upper level (B or C). 



Specific characters. — Skull proportions of males 

 more robust than in A. serotinus. Skull length 750 

 millimeters, width 570 (estimated). P'-m^ 370 

 millimeters. Nasals broad and abbreviate, horns 

 massive and obtuse. A small pair of lateral incisors. 

 Canines as in A. serotinus. Superior premolars with 

 distinct tetartocones and well-defined external cin- 

 gula. Tetartocone on p' distinct but somewhat 

 smaller than the deuterocone. Hypocone wanting 

 on m', replaced by rudimentary metaloph. Zygo- 

 mata widely arched outward, with buccal expansions. 



General characters. — The type (Nat. Mus. 4289) of 

 this species resembles that of A. serotinus (1) in the 

 peculiar lanceolate form of the canines, which are elon- 

 gate, compressed anteroposteriorly, and flattened on 

 the posterior face, perhaps a further development of 

 a condition seen in A. marshi; (2) in the marked 

 development of the cingulum on the premolars; 

 (3) in the proportions of the premolars; (4) in the 

 trihedral basal horn section. The type differs dis- 

 tinctly from A. serotinus (1) in the far greater develop- 

 ment of the tetartocones; (2) in the larger and more 

 robust proportions of the skull; (3) in the short 

 obtuse horns. Its structural character, like that of 

 A. serotinus, is intermediate between those of the 

 Brontops and Menodus main phyla, and in its extreme 

 size it perhaps presents an advanced stage of evolution 

 of the subgenus Allops — that is, of the Allops col- 

 lateral phylum — in the same manner that Diplo- 

 clonus amplus presents an extreme evolution of its 

 collateral phylum. It is noteworthy that this skull 

 differs from all the preceding male and female skulls 



