SCOTT: TOXODONTA OF THE SANTA CRUZ BEDS. 1 83 
He adds that 95 per cent, of the specimens from the Santa Cruz beds are 
referable to N. imbricatus (’94*, 244). While it is perfectly true that 
differences in the form of the cranial bones may often be used for the 
distinction of species, yet this criterion is practicable only when the dif- 
ference is relatively constant. In the present instance, not only is the 
character a fluctuating one, but the two extremes are connected by inter- 
mediate gradations, the nasal processes of the frontals varying much in 
size to complete disappearance. On the other hand, it may be said that, 
as a whole, the genus Nesodon is characterized by the presence of large 
nasal processes of the frontals and Adinotherium by their absence, though 
in both genera exceptions are not rare. 
Much the same reasoning applies to the supposed species, N. obliteratus, 
which is defined by the absence of the lower canine, the crowded condi- 
tion of the lower teeth, the less procumbent inferior incisors and the more 
steeply inclined symphysis. (Ameghino, ’94*, 245.) If they were con- 
stant, these characteristics would be quite sufficient to distinguish the 
species, but, as a matter of fact, they are fluctuating. 
Between the two species of Nesodon which are here recognized, the 
only trustworthy distinction is that of size, N. imbricatus being the larger 
and N conspnrcatus the smaller. There is, of course, considerable varia- 
tion in size in both of these species, but they do not seem to intergrade, 
although there is as great a difference of size between the largest and the 
smallest individuals of N. imbricatus as between the latter and the larger 
examples of N conspnrcatus. In many cases these variations in size are 
due to differences of age and sex, but there are also marked differences 
between individuals of corresponding ages. In N. imbricatus the upper 
profile of the skull is typically of a moderate convexity in the antero- 
posterior direction, but not infrequently there is quite a steep ascent from 
the forehead to the occipital crest, as in the skull figured in Plate XIII. 
The forehead itself is nearly plane in the fore-and-aft dimension and is 
usually a little depressed below the level of the nasals. 
In the following table the measurements of a representative series of 
individuals are given and they bring out clearly the changes in the form 
and size of the teeth with advancing age, the columns being arranged 
with the younger individuals on the left and the older on the right, 
though Nos. 15,135 and 15,252 are of nearly the same age and the 
former is probably a female. 
