SCOTT: TOXODONTA OF THE SANTA CRUZ BEDS. 
219 
of distinguishing contemporary and fluctuating variations from successive 
and relatively constant mutations and, until that distinction can be made, 
nothing more than a tentative arrangement of species is practicable. 
Ameghino has, it is true, given some valuable stratigraphical notes upon 
the vertical range of the species, but these notes are far from exhaustive 
and they do not entirely agree with the results obtained by Messrs. 
Hatcher, Peterson and Brown. Before full and trustworthy evidence 
upon these points can be obtained, it will be necessary to make very 
much larger collections than have yet been gathered and, above all, to 
make these collections together with a minutely accurate stratigraphical 
survey. Until then, the problem of species will be incapable of satis- 
factory solution. 
In his latest publication on this genus (’07, 67-89) Dr. Ameghino 
recognizes six species of Adinotherium, A. magister , A. ovinum, A. niti- 
dnm , A. corriguenense, A.ferum and A. robustiim, to which should be added 
two species referred to other genera, the so-called Acrotherium karaikense 
and Noaditherium splendidwn. A. haplodontoides Amegh. (’ gi b , 129; 
gi h , 376; 94", 25) is not mentioned, even in the synonymy. The six 
species enumerated are arranged in two series, in accordance with the 
presence or absence of the incipient frontal horn. An obvious suggestion, 
which almost forces itself upon the observer, is that this minute horn is a 
sexual rather than a specific character, especially as the horned skulls are 
almost always larger and more robust and have more prominent and 
rugose crests and processes, all of which are characteristically male 
features. Of course, this suggestion does not necessarily imply that some 
of the species may not have been hornless in both sexes, for that would 
be quite compatible with the presence of horns in the males of other 
species, as appears to have been the case in Nesodon. In that genus only 
one of the known species, N. cornutus, distinguishable as such upon other 
grounds, shows any indication of the frontal horn, but, within the limits 
of the species, the horn may well have been a sexual character. At 
present, however, there is no way of deciding whether this is true or not. 
Ameghino attaches much importance to the form of the fronto-nasal 
suture for the discrimination of species in both Nesodon and Adinotherium , 
but I have found this character, in the latter genus as well as in the former, 
too fluctuating to be trustworthy. Broadly speaking, there is a distinction 
between the two genera in regard to the form of this suture ; in Adinothe- 
