HISTORY OF THE fTOXODONTIA 471 



^Toxodon, or whether it was merely one of a short-lived branch 

 from the main stem, in which the teeth had acquired an un- 

 usual degree of complexity. 



A few years ago Dr. Ameghino announced the very sur- 

 prising discovery that, instead of having merely the normal 

 arrangement of two dentitions, the milk and the permanent, 

 ^Nesodon developed three successive dentitions, one preceding 

 the milk-series, and therefore called pre-lacteal. In certain 

 other mammals traces of a pre-lacteal series had already been 

 found, in the shape of tooth-germs, which never attain full 

 development or even cut the gum ; and quite recently Dr. 

 Ameghino has shown that in the tapir at least one functional 

 pre-lacteal premolar is formed. The significance of this fully 

 developed pre-lacteal dentition in fNesodon is not yet clear, 

 though it seems reasonable to suppose that it was the almost 

 uniquely late retention of a primitive character. 



The skull was closely similar to that of \Toxodon, on a 

 smaller scale, but there were several minor differences, which 

 were, in part, conditioned by the larger and much more com- 

 pletely hypsodont teeth of the Pampean genus, as well as by its 

 generally increased size and bulk. In ^Nesodon the sagittal 

 and occipital crests were much more prominent and the former 

 was much longer, while the thickening of the cranial bones 

 was in only an incipient stage. The nasal bones were consider- 

 ably longer. The jaws were lower and shallower, in correla- 

 tion with the less perfectly hypsodont teeth, and in the lower 

 jaw the chin was much more erect and rounded. The entire 

 head of this curious Santa Cruz animal had something 

 remarkably rodent-Uke in its appearance, though it is 

 quite inadmissible to suppose that the likeness was due to 

 relationship. 



The skeleton was far smaller and hghter and otherwise 

 differently proportioned from that of ^Toxodon, but there was, 

 nevertheless, a close agreement between the two genera. The 

 neck was of moderate length and thickness, the body long and 



