Order TOXODONTIA. 
The exigencies of serial publication have made it necessary to defer the 
account of the order until the descriptions of the three suborders had 
been completed. As here employed, the term Toxodontia is equivalent 
to Roth’s Notoungulata, which has found a wide acceptance, but I prefer 
to use the older term and to regard as suborders its three very distinctly 
marked subdivisions. It may well be that the number of subordinal 
groups should be increased for the reception of certain of the more 
ancient genera and families, such as the Notohippidae, but it would 
be premature to do so until much more complete material shall have been 
obtained. 
While the essential plan of structure is the same throughout the order, 
each of the three suborders, Toxodonta, Typotheria and Entelonychia, 
has its well defined and characteristic modifications, which affect all parts 
of the organism. The Toxodonta are large and moderate-sized animals, 
becoming extremely massive in the Pampean and immediately preceding 
formations, but there are quite small animals of this group in the 
Santa Cruz. In the Deseado stage, when they reached their culmination, 
the Entelonychia had some gigantic members and in the Santa Cruz 
most of the species are large animals. The Santa Cruz Typotheria, on 
the contrary, are all small creatures, some, like Pachyrnkhos, very small, 
and even in the Pampean they attain to no great size. 
Dentition. — In the Santa Cruz and subsequent formations the Toxo- 
donta and Typotheria have a strong tendency to hypsodontism, which in 
the latter is already fully established in Santa Cruz times, and the upper 
molars are very strongly curved, almost meeting in the median line of 
the palate. The Santa Cruz Toxodonta have high-crowned but rooted 
premolars and even the molars develop roots in old age ; in the subse- 
quent geological stages the teeth are completely hypsodont, but the Leon- 
tiniidae, of the older Deseado stage, which in my judgment should be 
referred to the Toxodonta, have brachyodont teeth. The Entelonychia 
are all brachyodont, though an increase in the height of the crowns may 
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