PLAGIARTHRUS 63 
by the external and internal folds which almost meet in 
the median line. On the outside, each tooth is coated with 
a layer of cement. When better known it may prove that 
this genus, so specialized in the character of the teeth, 
does not belong in this family. 
Plagiarthrus clivus Ameghino 
P. clivus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 436. 
This species is represented by a single specimen from 
the Chico del Chubut, west of Puerto Visser, which pre- 
serves pm. 3 and 4 and the molars. 
The characters of this, the type 
Seecies) sare: those Of the "genusi yoo von igve: emolare 9 and 
he: tocal length’ of the fiveteeth isi) 42nd molars 2s natural size. 
36 mm., and fig. 28 shows in natural size the various in- 
dividual teeth. 
Hegetotheriidae Ameghino 
This family includes a large variety of forms from the 
formations from the Deseado up to the Mt. Hermosa, but 
all agree in having the first upper and the first two lower 
incisors enlarged into strong gnawing teeth; in the reduc- 
tion or absence of in. 3, the canine, and premolar 1 of the 
upper and lower dentitions; in having the external face of 
the upper molars not inflexed; in lower molar 3 being three- 
lobed; and in the bulla being inflated and hollow. There 
are in the family two series of forms, at least, the one lead- 
ing to the running Zegetotherium, the other to the hopping 
Pachyrukhos, and the very little known form Phanophilus 
which may fit into one of the other series when better 
known. 
In the Deseado the following genera are assigned to 
the family. 
Prohegetotherium, like Hegetotherium, except that the 
last premolar and the molars have a vertical furrow near 
the external anterior margin. 
