EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



573 



the flatness of the anterior face of the horn section 

 (fig. 473). At the sides of the horns there is a short 

 extension of the external crest, and in this individual 

 the malar, anterior, and intei'nal angles are not very 

 sharply defined. Characteristic of the horns are 

 the external ridge extending from the sides of the 

 nasals upward and the very marked hollowing out 

 of the maxillary face. As observed in the original 

 description the great size of the maxillary horn pillars 

 reduces the opening of the anterior nares to a narrow 

 aperture. The connecting crest is fairly prominent. 

 The vertex of the skull has the very characteristic 

 longitudinal uniform breadth. The occipital crests 

 are wanting, but the paired knobs (fig. 378, F) are 

 preserved. As in Brontotherium and Megacerops 

 the basisphenoid is without rugosity. As in brachy- 

 cephalic skulls generally the postglenoid and post- 

 tympanic processes are widely conjoined, the auditory 

 meatus being very small. 



Teeth. — The upper premolars are distinctly of the 

 Brontotherium- Megacerops type, with well-rounded, 

 distinct tetartocones and reduced cingula. 



Conclusion. — This skull differs from the type of 

 B. (Titanops) medium (Marsh), in its smaller size 

 and in the absence of the basisphenoid rugosity. It 

 exhibits some resemblances to Megacerops acer but is 

 distinguished by the more marked divergence of the 

 horns. In the absence of knowledge of the occiput and 

 of the condition of the cutting teeth, incisors, and 

 canines, the phyletic position of this species can not 

 be considered as absolutely determined at present. 



It appears possible, however, that in spite of its 

 somewhat smaller size (compare the range in size in 

 skulls referred to B. gigas) this species may be the 

 same as that later termed B. medium by Marsh. 



Brontotherium medium (Marsh) 



(Titanops medius Marsh, 1891) 



Plates XIX, XX, CLXXII, CLXXVI, CLXXX; text figures 

 25, 191, 395, 407, 471, 475 



[For original description and type references see p. 228] 



Geologic horizon. — Upper Titanotherium zone of 

 South Dakota. 



Specific characters. — Size about that of a large B. 

 gigas; basilar length of skull 825 millimeters; grinding 

 series elongate (365 mm.), dental index 43 (about as 

 in B. gigas); nasals more reduced (free length 45 mm., 

 free breadth 110); horns long (320 mm.), becoming 

 flattened in section, connecting crest in type shal- 

 lower than in B. gigas or in B. curtum. 



So far as we can observe this species appears to be 

 in a stage of evolution very similar to that of the 

 previously described B. dolichoceras. 



The name "medius^' was probably assigned by 

 Marsh in reference to the intermediate length of the 

 nasals between those of B. gigas and B. curtum. 



Materials. — Our knowledge of this evolution stage 

 is afforded by the type specimen of B. (Titanops) 

 medium, a large and finely preserved male skull in the 

 United States National Museum (No. 4256). It is 

 recorded as from the top levels of the upper Titano- 

 therium zone. The grinding series in the type speci- 

 men of B. medium is exceptionally elongate, namely, 

 365 millimeters; consequently we have associated 

 with this species a very large lower jaw (Am. Mus. 

 1051). 



Slcull. — We notice especially the persistence of two 

 upper incisors and the prominence of the well-rounded 

 tetartocones on the premolars, that upon the right, p"*, 

 being sharply distinct. The incisors differ from the 

 typical brontothere form in exhibiting smooth, rounded 

 crowns, apparently noncingulate. The canines are 

 short (38 mm.), heavy, and blunt, as in B. gigas, with 

 robust posterior cingula. The anterior premolar, p', 

 is triangular and exceptionally small, with a single 

 much-worn internal cusp. The hypocone of m^ is 

 small and cingulate, but from this cusp there extends 

 completely across the crown the abortive metaloph 

 so characteristic of many members of this and other 

 phyla. The animal is still young, being intermediate 

 between the fifth and sixth stages of growth ; the only 

 internal cones which are worn are those of m.\ yet the 

 horns attain an outside measurement of 320 millime- 

 ters, proving the rapid individual development of these 

 important organs. The horns are distinguished by 

 the sharp carrying up of the external and anterior 

 ridges to a high point. Thus the outer or maxillary 

 face is concave below and flattened above. This fea- 

 ture is also seen in a less conspicuous manner in the 

 type of B. dolichoceras. The nasal section and the 

 length of the nasals (45 mm.) are also identical in the 

 two specimens. 



Lower jaw. — Possibly belonging to this species is the 

 enormous jaw in the American Museum collection 

 (No. 1051). As shown in Plate CLXXII, B, and in 

 Figure 471, B, this has the characteristic form of the 

 canines, the posterior molar, angle of the jaw, etc., of 

 B. gigas, but the lower incisors show some reduction 

 and lack the decided development of the cingulum 

 observed in No. 1070 (B. hatcheri). Moreover, the 

 great length of this jaw (730 mm.), measured from the 

 condyle to the symphysis, favors its reference to 

 B. medium rather than to B. ramosum or B. platyceras, 

 in which the jaw is relatively shorter. 



Conclusion. — B. medium is one of several stages of 

 the upper zone resting on a single type and tending 

 to connect B. hatcheri, B. gigas, B. dolichoceras, 

 B. medium, B. curtum, B. ramosum, B. platyceras in a 

 more or less continuous phjdum. 



Additional observations on Brontotherium medium. — 

 The type skull is remarkable for the large size of the 



