SCOTT : TOXODONTA OF THE SANTA CRUZ BEDS. I 1 7 
shall be to demand recognition as a distinct family is, of course, largely 
a matter of individual judgment, and perfect agreement on this point is 
hardly to be expected. If classification is to be the concrete expression of 
genetic relationship, the second method is clearly the better and more 
logical one. For example, all of the horses, from the lower Eocene to the 
present, are by this method included in the Equidae, both the genera 
which can be placed in the main line of descent and such side-branches as 
Anchitherium, Hippidium, etc., as are given off at different levels from 
the principal stem. On the other hand, if, as is probably true, the Palae- 
otheriidae are derivatives of the same stock, they diverge and ramify so 
much as to form a separate family. 
An even more instructive example is that of the rhinoceroses, of which 
Osborn (’ i o, 557-8) recognizes two families, the Hyracodontidae and 
Rhinocerotidae, and divides the latter into four subfamilies. Personally, 
I prefer the classification which includes all of the rhinoceroses in one 
family, dividing this into three subfamilies, for the true rhinoceroses, the 
cursorial Hyracodonts and the presumably aquatic Amynodonts respec- 
tively, but Osborn’s arrangement will suffice for the comparison. The 
family of the Rhinocerotidae includes much more diversified forms than 
are known among the Toxodonta. Whether we consider the dentition, 
the skull, or the feet, we find far greater differences between such genera 
as the Oligocene Trigomas, the Miocene Teleoceras and the Pleistocene 
Elasmotherium than can be found among the known Toxodonta. To 
this single family are referred animals with and without horns, with single 
and with paired nasal horns, with frontal horns present or absent, with 
and without incisors and canines, with grinding teeth brachyodont and 
comparatively simple, or hypsodont, cement-covered and highly complex, 
with tetradactyl or tridactyl feet, which are either long or slender, or short 
and extremely heavy. Comparing the rhinoceroses with the toxodonts, 
I can see no valid reason for making more than one family for the Santa 
Cruzian and later members of the suborder. 
From the foregoing considerations the Notohippidae have been excluded, 
because I have had but little opportunity to examine the animals of this 
group and they are still incompletely known, but they are doubtless enti- 
tled to separate family rank. The same is true of the Leontiniidae of the 
Pyrotherium beds, which are so aberrant that they are usually included 
in the Entelonychia. I am confident, however, that this reference is 
erroneous. 
