492 The American Naturalist. [June, 
The incisor teeth display an increased specialization in the 
genera commencing with Trigonodon and ending with Toxo- 
don. The dental formula in Toxodon is I. 3; C.~; Pm. 3; 
M. 3; in Trigodon it is I. .3; C.1; Pm.3; M. 3. In Trigono- 
don the median (4 2) incisors are smaller and have closed 
roots, while the external is large and has open roots. In Tox- 
odon the I. + has disappeared, and the I. 2 is extended trans- 
versely and has an open root. The I. 2 is narrower and more 
elongate and has an open root. The species of Toxodon differ 
as to the transverse extent of the I. 2; it being wider in the T. 
burmeisteri Gieb. than in the typical species, T. platensis Owen, 
and widest in the T. expansidens Cope from Brazil. 
The skull of Toxodon is wide and elevated posteriorly and nar- 
row anteriorly. The occiput slopes anteriorly and is notched on 
each side by a large mastoid foramen, somewhat as in the 
Sirenia. The nostrils have a posterior position. For these 
reasons it has been suspected that there may be some affinity 
between the Toxodontia and the Sirenia. 
The species of Toxodon were, according to Ameghino, heavy 
animals with rather. short legs, the anterior the shorter. He 
imagines that they were shore dwellers or semi-aquatic, in some 
degree like the Hippopotamus in their habits. The 7. platen- 
sis Ow. is about the size of the Rhinocerus unicornis; the T. 
burmeisteri is somewhat larger, while the T. ensenadensis is of 
still larger dimensions. The Toxodontotherium compressum 
Amegh. was of about the size of the typical Toxodons, while 
the species of the other genera of the family are of successively 
smaller size, those of Dilobodon being the least. 
