INTRODUCTION. 
Tux following monograph consists essentially of two quite distinct parts. 
The first embraces descriptions of the species, while the second is devoted to 
the consideration of the geographical distribution of Bison americanus. The. 
first part of Part I, or that portion relating to the extinct species, is the least 
satisfactory, owing to the scantiness of the materials for their description. 
It has been my good fortune, however, to have the opportunity of examining 
nearly all the material thus far described from the United States relating to 
these interesting forms, including the original specimens of Bison datifrons and 
Bison antiquus, as well as the later-described bison remains from California. 
The specimens from Eschscholtz Bay, described by Richardson, I have not 
had an opportunity of seeing, but I have had access to a few remains of © 
the extinct bison from other Alaskan localities. 
In the following pages two extinct species are recognized, which differ 
quite widely from each other, the one (Bison latifrons Leidy) being much 
larger than the other (Bison antiquus Leidy = Bison crassicornis Richardson), 
with disproportionately larger horn-cores. Neither of the species is as yet 
known from satisfactory material, although enough of their remains have 
been found to indicate the existence of two widely differing forms. Without 
knowing positively more of Bison datifrons than the three cranial fragments 
thus far discovered represent, it is difficult to assign some other specimens 
to the one species rather than to the other, owing to our lack of knowledge 
of the sexual difference in size this large species may present. The female 
of the larger extinct species, judging from the sexual differences seen in the 
living species, would apparently about equal in size the male of the smaller 
one, and hence it is difficult to positively specifically assign such specimens 
as detached teeth or single bones of ‘the extremities. Again, the female of 
the smaller extinct species being of about the size of the male of Bison 
americanus (perhaps a little smaller) still further complicates the problem. _ 
