SCOTT: TOXODONTA OF THE SANTA CRUZ BEDS. 1 3 1 
Stage 10 . — Animal has attained its full development. I 1 has decreased 
in size by abrasion, while i~, growing from a persistent pulp, is longer 
and much heavier than i 1 . Mi is a little larger than m^, a much more 
marked difference than in the preceding stage. Upper molars decreased 
in antero-posterior diameter through abrasion and of more square shape ; 
roots begin to appear on the molars and those of the premolars to close. 
Sagittal crest high and thin. This stage corresponds to the genus Neso- 
t her him of Mercerat. 
Stage 11 . — This stage is well distinguished by the relatively enormous 
size of m- in proportion to the other teeth ; all the molars have the antero- 
posterior diameter reduced by abrasion and the upper molars have a cross- 
section of trapezium-shape ; all the molars, except m-, have well-formed 
roots ; mi has one long root, but open and undivided. Sagittal crest very 
high and compressed. This stage corresponds to the genus Protoxodon 
as defined by Mercerat. 
Stage 12. — Mi is proportionally very large and the other teeth much 
smaller ; at a still more advanced age m- forms several roots. 
The Pampean genus Toxodon has a dentition which may be described 
as that of Nesodon somewhat reduced in number and considerably simpli- 
fied in pattern, but thoroughly hypsodont, all the teeth growing from per- 
sistent pulps. Lydekker’s brief statement may be here quoted with 
advantage: “This genus ... is characterized by a somewhat Rodent 
like dentition, in which all the teeth grow from persistent pulps, the 
normal dental formula in the adult being if, cf, pf,* mf. The first upper 
premolar may, however, frequently be wanting in the adult, and occasion- 
ally the corresponding lower tooth is present. The upper molars are 
characterized by their height and decidedly triangular form, the middle 
lobe being but slightly developed and almost disappearing in an advanced 
state of wear, while the main cleft on the inner side continues to the base 
of the crown even in the most advanced state of wear and there is no 
posterior valley. The lower molars are very narrow, with three internal 
folds, the third tooth of this series being much larger than either of the 
others. In both jaws the premolars are much simpler than the molars. 
Both pairs of upper incisors are scalpriform, and while the first and second 
* Lydekker has z /i, an obvious misprint, as is shown by the statement immediately following. 
