EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



457 



the tips of the nasals; (3) loss of variable and vestigial 

 teeth, incisors and premolars, in old age; (4) union of 

 the anterior caudal vertebra with the sacrum to form 

 four sacrals (Hatcher, 1893.1, p. 217). 



In general, growth is in a high degree differential — 

 that is, proportions constantly change, as, for example, 

 in the gain of width over length, in the rapid increase 

 of the horns in length, and in the relative gain in the 

 length of the nasals, although only in a few specimens 

 have we sufficient material to measure these differ- 

 entials in growth. 



Abnormal sport and reversional characters. — Among 

 the abnormal characters may be observed the following : 

 (1) Reversional or abortive protoloph and metaloph 

 on the superior premolar teeth; (2) abnormal redupli- 

 cation of tetartocones on superior premolar teeth ob- 

 served in specimens of Brontops dispar, Allops cras- 

 sicornis, Menodus varians; (3) progressive rectigrada- 

 tional or anomalous reduplication of horns as observed 

 in specimens of Diploclonus, Menodus, Brontotherium. 



SECTION 3. DIVISION OF THE OLIGOCENE TITANO- 

 THERES INTO GROUPS AND SUBFAMILIES 



CHARACTERS OF THE SKUII AND TEETH OF THE MENO- 

 DONTINE AND BRONTOTHERIINE GROUPS 



The following study of the characters of the skull 

 and teeth enables us to divide all the highly varied 

 forms of Oligocene titanotheres into two great groups, 

 the menodontine and the brontotheriine, which sepa- 

 rated from each other in Eocene time. (See p. 467.) 

 These characters, which are much more pronounced in 

 male than in female skulls, are seen to be the direct 

 and indirect results of one or other of the opposing 

 principles of skull and tooth transformation described 

 on pages 254-262, 450, namely: 



1. Dolichocephaly and dolichopy versus brachy- 

 cephaly and brachyopy; cyptocephaly. 



2. Differential transformation of canines (p. 448), 

 premolars, and molars. (See figs. 405, 406.) 



3. Differential development of horns, nasals, orbits, 

 zygomata, etc. (See figs. 389-394.) 



Primary groups oj the Oligocene titanotheres 



MeDodontine group (Teleodus, Brontops, Diplo- 

 clonus, Allops, Menodus) 



Brontotheriine group (Megacerops ("Symborodon"), 

 Brontotherium) 



Canines - 



Opposite grinding series as seen in psflate 



view. 

 Upward flexure of premolars as seen in 



side view. 

 Antorbital region of skull 



Premolar series (length) 



Internal cusps of upper premolars 



Ectolophs of grinding teeth 



Hypocone of m^ 



Internal cingula 



External cingula 



Zygomata 



Horns 



Malar-Iacrimal bridge over infraorbital 



foramen. 



Anterior narial cavity 



Backward prolongation of occiput behind 



zygomata. 

 Jaw 



Pi 



Convexity, top of parietals 



Orbit 



Skull vertex 



Skull vertex, side view 



Proportions of molar teeth 



Primitively pointed, recurved, not closely 

 approximated toward median line. 



Not strongly arched anteriorly toward 



median line. 

 Slight to moderate 



Relatively elongate 



Relatively longer 



Steep-sided (Menodus); moderately so 

 (Brontops) . 



Somewhat more vertical 



Often surrounded by cingulum 



Sharp on premolar teeth 



Usually pronounced 



Slight to heavy, often deep 



Short, diverging obliquely outward and 



forward . 

 Usually very broad 



Broad and open 



Moderate 



Angle sharply produced backward 



Somewhat more elongate 



Absent 



Medium to large 



Divergent anteriorly, convergent pos- 

 teriorly. 



More concave 



More elongate 



Primitively short, bulbous, with swelling 

 posterior cingulum, closely approxi- 

 mated. 



Strongly to very strongly arched toward 

 median Hne. 



Very pronounced. 



Abbreviated (Brontotherium) to very 



abbreviated (Megacerops) . 

 Relatively shorter. 

 Very low, robust, subcircular. 



Sharply depressed to crown. 



Very heavy, triradiate, continuous with 

 cingulum. 



Often less developed and rounder. 



Reduced or absent. 



Broad to extremely expanded and flat- 

 tened. 



Long, usually more erect. 



Usually very narrow. 



Becoming very high and narrow. 

 Moderate to extreme. 



Angle less produced backward, more 



slender. 

 Somewhat abbreviate. 

 Pronounced. 

 Medium to small. 

 More or less parallel. 



More convex. 



More expanded transversely. 



