20 



TITANOTHEEES OP ANCIENT WYOMING, DAKOTA, AND NEBEASKA 



Oligocene stages of titanotheres of the Brontotherium 'phylum in the Titanotherium zone 



" Genus Titanops IVIarsli. 

 ZOOLOGIC AND PALEONTOLOGIC NOMENCLATURE 



Significance of the table. — The sequence shown in the 

 accompanying table, which presents what is believed 

 to be a generic, monophyletic, or nearly single phyletic 

 series of changes of form, evolving in a single geographic 

 region of South Dakota, illustrates the manner in 

 which the Linnaean binomial system and the muta- 

 tion substages of Waagen may be adapted to express a 

 phyletic sequence. The newer trinomial names of 

 modern mammalogy and the subspecific names may 

 be employed to connect the intergrading mutations. 



The most primitive species, Brontotherium leidyi, is 

 so notably distinct in size and skull structure from the 

 most advanced species, Brontotherium platyceras, that, 

 if named by zoological standards, it might well be 



' Type of genus Brontotherium (Marsli). 



placed in a separate genus — in fact, several generic 

 names have been suggested for members of this 

 phylum, namely, Brontotherium, Titanops, Bronto- 

 theridion (MS.) — but the subdivision of such a phylum 

 into a number of genera would obscure the all-im- 

 portant monophyletic unity, for such a phyletic genus 

 is defined by its peculiar and distinct evolutionary 

 tendencies. For example, the genus Brontotherium 

 tends toward the evolution of flattened horns, a charac- 

 teristic which begins in a very slight flattening of the 

 posterior side of the horn, as observed in B. leidyi, and 

 develops into the extraordinarily broad, flattened 

 horns of B. platyceras. 



New phyletic meaning of species. — The species repre- 

 sented by large collections of mammals like those of 

 some of the phyla of the titanotheres, especially the 



