ISOTEMNIDAE
This family is distinguished by the formula
3 1 4 3
-------,
3 1 4 3
by the incisors, canine and premolar 1 all being of subequal size,
by all the teeth being brachydont, and by the crescents of the lower
premolars and molars being modified. On these lower premolars and
molars the anterior crescent is longer than the posterior, and the
short posterior crescent on the exterior of the tooth; so that its
anterior horn, instead of uniting with the posterior horn of the
anterior crescent, comes in back to about the middle of the anterior
crescent. Then the pillar, which in the other families is situated in
the posterior crescent, is opposite the posterior horn of the posterior
crescent. The small animals which represent this family are rare in the
Deseado beds, much more abundant in the Casamayor. The family seems to
have died out in the Deseado as no forms are referred to it in later
epochs. We found no specimens belonging to the family; but to make this
discussion complete, I will give a digest of Ameghino’s descriptions,
with reproductions of such figures as he has given. All of the genera
and species are based on very fragmentary material.
The genera assigned to the family are _Trimerostephanos_,
_Pleurocoelodon_, _Lophocoelus_ and _Henricofilholia_.
=Trimerostephanos= Ameghino
Trimerostephanos Amegh., 1895, Bol. Inst. Geog. Argen., t. 15, p. 646.
Trimerostephanos Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 483.
This genus is based on upper and lower teeth, and distinguished by the
premolars and molars having a weak style on the anterior corner, and by
the anterior lobe being considerably larger than the posterior. Four
species have been described.
=T. scabrus= Amegh., loc. cit. for genus.
This is the type species, originally based on the third lower molar, to
which was later added, upper premolar 4 and the molars, and lower molar
2. The following measurements are given:
Upper premolar 4, length 15 mm., width 21 mm.
Upper molar 2, length 31 mm.
Upper molar 3, length 35 mm.
Lower molar 2, length 20 mm.
Lower molar 3, length 29 mm.
[Illustration: Fig. 87. T. scabrus—natural size; _A_, upper premolar 4;
_B_, lower molars 1 and 2.]
=T. scalaris= Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 483,
is based on lower pm. 2 to m. 2, a somewhat smaller species than the
preceding, the series as given measuring 53 mm.
[Illustration: Fig. 88. T. scalaris, premolar 2 to molar 2—natural
size.]
=T. angustus= Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 484.
This species is described without a figure, as smaller than _T.
scalaris_, pm. 2 to m. 2 being 59 mm. The mandible is also slenderer.
=T. biconus= Amegh., 1897, Bol. Inst. Geog. Argen., t. 18. p. 484.
This species is based on two lower premolars, said to be the same size
as _T. angustus_, but with the pillar larger.
=Pleurocoelodon= Ameghino
Pleurocoelodon, Amegh., 1895, Bol. Inst. Geog. Argen., t. 15, p. 645.
Pleurocoelodon, Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 484.
This genus is distinguished by the absence of the style on the anterior
external margin of the upper molars, instead of which the external face
is excavated medianly. Two species are described, based on isolated
upper teeth.
[Illustration: Fig. 89. P. wingei—natural size; _A_, first molar; _B_,
third molar.]
=P. wingei= Ameghino
P. wingei Amegh., 1895, Bol. Inst. Geog. Argen., t. 15, p. 645.
P. wingei Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 485.
This species is founded on a couple of isolated molars, probably
belonging to the same individual. The following measurements are given:
Upper molar 1, length 22 mm., width 26 mm.
Upper molar 3, length 24 mm., width 29 mm.
=P. cingulatus= Amegh., loc. cit. above, is based on an incomplete
upper molar, probably the second, which is distinguished by having the
internal cingulum excessively developed. It measures 30 mm. in length.
=Lophocoelus= Ameghino
Lophocoelus Amegh., 1904, Anal. Soc. Cient., Rep. Argen., t. 58, p. 245.
Lophocoelus Amegh., 1904, Anal. Mus. Nac., ser. 3, t. 3, p. 352.
The genus is founded on a single upper third molar from Mazaredo,
which is distinguished by a feeble style on the external face, by the
anterior lobe being obliquely placed, and by the presence of a small
secondary bay on the posterior side of the great internal basin.
=L. macrostomus= Amegh., loc. cit. above.
This species is the only one described, and has the generic features,
the upper m. 3 being 21 mm. long, by 25 mm. wide.
=Henricofilholia= Ameghino
Henricofilholia Amegh., 1901, Bol. Acad. Nac. Cordoba, t. 16, p. 404.
The type species is _H. cingulata_, based on a single upper molar. In
general the upper molars are similar to those of _Leontinia_, but more
brachydont, and with the internal cingulum well developed and tending
to be crenulated. Four species have been made, all based on isolated
upper molars.
=Henricofilholia cingulata= Ameghino
H. (? Parastropotherium) cingulata Amegh., Bol. Inst. Geog. Argen., t.
15, p. 640.
H. (? Parastropotherium) cingulata Amegh., 1897, Bol. Inst. Geog.
Argen., t. 18, p. 450.
H. cingulata Amegh., 1901, Bol. Acad. Nac. Cienc. Cordoba, t. 16, p.
404.
[Illustration: Fig. 90. H. cingulata, upper molar 1—natural size, after
Ameghino.]
This is based on an upper molar 1 of which I reproduce Ameghino’s
figure. It measures 28 mm. long by 29 mm. wide.
=H. lustrata= Amegh., 1901, Bol. Acad. Cienc. Cordoba, t. 16, p. 405.
This species is smaller than the preceding, and is based on an upper
molar 1 and a last lower molar. The measurements are as follows:
Upper molar 1, length 25 mm., width 25 mm.
Lower molar 3, length 25 mm., width 12 mm.
[Illustration: Fig. 91. H, inaequilatera, upper molars 3 and 4—natural
size, after Ameghino.]
=H. inaequilatera= Amegh. loc. cit. above.
This species is larger than the preceding with the internal cingulum
more developed. Upper molar 2 measures 30 mm. long by 29 mm. wide.
=H. circumdata= Amegh., loc. cit. above.
This is a still larger type, with the internal cingulum enormously
developed. Upper molar 1 measures 42 mm. long by 36 mm. wide.