Sinclair: marsupialia of the santa cruz beds. 
377 
pie, laterally compressed blade, with a rounded heel, but without heel cusp. 
The median premolar is a replica of the anterior tooth on a larger scale. 
The posterior premolar is enlarged, with a prominent, piercing protocone 
and a broad heel, with distinct heel cusp. The prominence of the latter 
varies with the amount of wear to which this part of the tooth crown has 
been subjected. The first, second and third molars have the protocone 
well developed. It is slightly cupped in unworn teeth. On M 1 , the pro- 
tocone is reduced to a small conical cusp, while the paracone and antero- 
external style are enlarged, and the metacone is vestigial. In the anterior 
molars, the proportions of the dental cusps are much the same as in Pro- 
thylacynus , except that the protocone of M- is well developed. 
In the inferior series, the incisor crowns show considerable lateral com- 
pression, but are so badly worn in all the specimens examined that their 
original shape cannot be ascertained. As in many recent carnivorous 
marsupials, the root of the second tooth in the series is displaced pos- 
teriorly with respect to the roots of the median and lateral incisors. The 
canines are reniform in cross-section owing to the presence of a broad 
groove on the lingual side. The anterior surface of the crown is flattened 
(PI. LVI, fig. i ; LIX, figs. 7, 7 a) apparently to accomodate the tooth to 
the narrow groove in the premaxilla, into which its point is received when 
the jaws are brought into occlusion. The amount of spacing between the 
lower premolars varies somewhat in different individuals of the same 
species, the greatest amount of variation in this respect occurring in the 
width of the space between the median and posterior premolars. The 
anterior and median premolars closely resemble the corresponding teeth 
in the superior series. Each consists of an enlarged piercing central cusp 
and a round heel. In unworn teeth, the heel of the median premolar sup- 
ports a small conical heel cusp. A minute anterior basal cuspule is also 
present. In worn teeth, the heel is reduced to a broad convex area, and 
the identity of the anterior cuspule is lost. The posterior premolar is 
enlarged like the corresponding tooth opposing it in the superior series. 
In unworn specimens a conical heel cusp is present. Characters of 
generic importance appear in the heels of the lower molars. The same 
rapid increase posteriorly in the size of the molar crowns is to be noted 
as in Borhycena and Prothylacynus. The cusps of the trigonid are iden- 
tical in shape and position with those of the latter genus. In the anterior 
molars, the heel supports a shallow, basin-shaped depression, bounded by a 
