SINCLAIR : TYPOTHERI A OF THE SANTA CRUZ BEDS. 
9 
theria in having the second incisor above and the third below greatly 
enlarged and caniniform, while in the Typotheria the median incisor in 
both jaws is the only one tending toward great increase in size. In none 
Fig. 6. 
A, Protypotherium sp., unworn Mg-, x | (No. 9482 American Museum) ; B , Nesodon imbri- 
catus, two lower molars, x f (No. 1 5 » 1 3 5 )• ac > anterior crescent; pc, posterior crescent; pp. 
posterior pillar. 
of the Santa Cruz Typotheria is there any trace of a double deciduous 
dentition as in Nesodon. 
From the resemblances in dentition and foot-structure already pointed 
out it may be inferred that the Toxodonta and Typotheria had a common 
origin, but the facts at present available do not justify a more extended 
discussion of relationships. 
II. With Typotherium. 
Difficult as it is to ascertain the relationship existing between the Santa 
Cruz Typotheria and the Toxodonta, it is even more so to determine their 
degree of kinship with Typotherium , the ancestor of which would appear 
to be Eutrachytherus , as suggested by Ameghino. From their small size 
it is quite probable that none are in the direct line of descent culminating 
in Typotherium , and this seems to be borne out by a consideration of the 
degree of specialization in dentition and foot-structure which the latter 
displays. Typotherium is more specialized in dental structure, showing 
greater complexity of folding in the molar crowns than is attained by any 
of its Santa Cruz predecessors, but less specialized in foot-structure, pos- 
sessing a pollex in the manus which has been lost in the most generalized 
of the Santa Cruz Typotheria ( Protypotherium ) and with digit V of the pes 
less reduced than in the most specialized of the latter ( Pachyrukhos ). A 
pollex is figured by Ameghino (1889, pi. 13, fig. 14) in the manus of 
Pachyrukhos ty pious, but none has been found in any Santa Cruz speci- 
