i8o 
PATAGONIAN EXPEDITIONS : PALEONTOLOGY. 
differently by different writers. Lydekker says of Nesodon : “It may, 
indeed, be regarded as the most generalized representative of the group 
with which we are at present acquainted, although it does not appear to 
have been the direct ancestor of Toxodon ” (’93, 25). If my memory 
does not deceive me, Dr. Roth expressed the same opinion to me in 
conversation, taking the ground that the more complex grinding teeth of 
Nesodon could not have given rise to the simpler molar-pattern of Toxo- 
don. On the other hand, Ameghino (’89*, 402; ’04, 219) appears to hold 
that the Pampean is directly descended from the Santa Cruz form. 
My own opinion is still undecided, though I am somewhat inclined to 
agree with the position taken by Ameghino. Dental evolution is not, as 
is often assumed to be the fact, always in the direction of increased com- 
plexity of pattern, but may result in simplification. Not to mention such 
groups as the Edentata and the Cetacea, we find among the Rodentia 
several clearly demonstrable instances, in which the acquisition or in- 
crease of hypsodontism is accompanied by a marked simplification of the 
molar-pattern, and I can see no insuperable difficulty in deriving the 
teeth of Toxodon from those of Nesodon. At all events, Nesodon very 
nearly represents the ancestor sought and may be used for all practical 
purposes of the comparison. Should it prove to be true that Nesodon is 
not the ancestor, it will, in all probability, be eventually shown that the 
ancestral and as yet unknown toxodont of Santa Cruz times very closely 
resembled Nesodon , differing from it chiefly in having a less developed 
post-tympanic region and possibly also in having simpler grinding teeth. 
In other regards, Nesodon is just what we should expect the ancestor of 
Toxodon to be. 
Species. — Though remains of Nesodon are individually among the 
most abundant of Santa Cruz fossils, the number of species represented 
is small. Indeed, only two species, of those hitherto described, can be 
definitely recognized, N imbricatus and N. conspurcatns (i. e. andium ), 
and these only by a constant difference of size. Many other names have 
been proposed, but they are either obvious synonyms, or of very doubtful 
propriety and until more abundant material has been gathered and espe- 
cially until the exact stratigraphic succession has been determined, it will 
hardly be practicable to make a satisfactory decision regarding some of 
these supposed species. 
