8 Bulletin American Museum of Natural History. [Vol. XXXIV, 
trigonid distinct from talonid, heel of mg longer with high hypoconulid and 
entoconid. There are no cingula on the molars and the posterior mental 
foramen is beneath m;. The upper molars show corresponding differences, 
being in Palgwosinopa triangular in outline, with high sharp protocone, 
hypocone represented only by a cingular flange, paracone and metacone 
smaller, sharper, somewhat inset from the border, and with small stylar 
crests at the anterior and posterior angles. The deuterocones of the pre- 
molars are also better developed, and the last molar is transverse. 
In all these characters of the teeth Palwosinopa comes decidedly nearer 
to the Miacide than does Thryptacodon. But the skeleton of the new genus 
is that of a Creodont, related to the Miacide and Arctocyonidee, whereas 
the skeleton of Paleosinopa is widely different from the Creodont type, and 
agrees nearly with that of the Insectivore Pantolestes. The distinctions in 
the teeth are therefore not to be regarded as of ordinal value. 
Two species or subspecies are represented in the collection, distinguished 
as follows: 
T. antiquus: m'*=17.5 mm.; upper molars round-quadrate, no hypocone on 
m’, no protostyles. 
T. olsent: m'*=21 mm., upper molars quadrate, distinct hypocone on all, 
protostyle on m!~?, deuterocone of p‘ larger. 
Thryptacodon antiquus sp. nov. 
Type, No. 16162, upper and lower jaws and parts of radius and ulna, from the 
Systemodon zone in Clark Fork Basin, Wyoming. 
Distinctive characters: Upper molars round-oval, with low rounded cusps, heavily 
cingulate, enamel rugose, distinct hypocone on m!~, none on m’, conules distinct but 
small. P%~ trihedral, three-rooted, strong deuterocone on p‘, none on p®. Lower 
molars broad, low cusped, with heavy external cingula, paraconid much reduced, 
submedian, eee and metaconid equal and opposite, heel wide-basined, with 
hy® and en‘ strong, wider apart on mi_»2 than the trigonid cusps, hypoconulid rudi- 
mentary except on m3, in which it is moderately large median-internal. Heel of 
m; and m* reduced in size. Posterior lower premolars short and robust with small 
heels and anterior basal cusps and heavy cingula. P» two- rooted, slender, p: one- 
rooted. Canines moderately large, slender, ridged posteriorly. 
Thirteen specimens from the Sand Coulée and Gray Bull horizons in the 
Big Horn and Clark Fork basins represent this species, all agreeing quite 
closely in size and characters. Two jaw fragments with m3 from the 
Clark Fork beds probably represent a distinct species or subspecies, dis- 
tinguished by broader teeth and a distinct protostyle on me, but the material 
seems inadequate for a specific type. 
