94 Bulletin American Museum of Natural History. [Vol. XVI, 



Brontotherium Marsh embraces the largest Titanotheres, 

 with very broad zygomatic arches, nasals shortening while 

 horns elongate and shift forwards; incisors persistent, |- in 

 the males, canines stout and obtuse. 



Representatives of Titanotherimn and Megacerops can now 

 be continuously traced from the base to the summit of the 

 Oligocene. Primitive species of Brontotherium also appear 

 at the base, although the phyletic sequence through the 

 Middle to the Upper Beds is not so clear. Symborodon sud- 

 denly appears in the Middle Beds. 



3. NEWLY OBSERVED PROGRESSIVE CHARACTERS. 



In addition to the progressive and retrogressive charac- 

 ters previously recorded by various writers (Osborn, 1896, 

 pp. 162-174), are several of great value in the determination 

 of species, (i) In the complication of the premolar teeth, 

 one or more phyla progress in common, although the rate is 

 unequal; for example, the double internal cones of the upper 

 premolars are precociously developed in Titanotherimn and 

 very gradually so in Megacerops. (2) The cranial and many 

 of the dental characters are profoundly affected by the pro- 

 gressive shortening or lengthening of the skull as recently 

 explained in some detail by the writer (Osborn, 1902). 



(3) The cingulum around the molar and premolar teeth be- 

 comes progressively stronger in Titanothermm and Megacerops, 

 and progressively weaker in Symborodon and Brontotherium. 



(4) The persistence or degeneration of incisor teeth are far 

 more constant sj^stematic characters than appeared to be 

 the case in my first review. 



In brief, each genus or phylum has its distinctive, persistent, 

 progressive, and retrogressive characters, of which the above 

 are a few examples out of many. 



4. THE SUCCESSION OF SPECIES. 



Phylum I. 



Dolichocephalic Titanotheres with short, divergent, triquet- 

 rous horns placed slightly in advance of the orbits; nasals elon- 



