SINCLAIR I MARSUPIALIA OF THE SANTA CRUZ BEDS. 
347 
BORHYAiNA Ameghino. 
(Plates XL-XLVI ; LII, Figs, i, 2 ,6; LIII, Figs. 2, 2 a, 4-4$, 9, 9 a; 
LIV, Figs. 7, 13 ; LXI, Fig. 3.) 
Borhyczna Amegh.; Enum. Sist. Especies Mamif. Fos. Patagonia Austral, 
p. 8, Dec., 1887. 
Dynamictis Amegh.; Revista Argentina Hist. Nat. I, entr. 3 a, pp. 148- 
149, June, 1891. 
A genus of large marsupial carnivores containing the most powerful 
predatory mammals in the Santa Cruz fauna. In the Princeton collection 
two species are represented by good skulls, with one of which a consid- 
erable portion of the skeleton is associated. 
Dentition (Pis. XL; XLII ; XLIV ; XLV, figs. 1, 3). — In both spe- 
cies ( B . tuberata , B. excavata ), the incisors are reduced to |, a formula 
unknown among existing marsupials, with the exception of Notoryctes. 
The upper incisors here have been worn to such an extent that the pattern 
of the crown is entirely obliterated. The median pair are laterally com- 
pressed and show no tendency to assume a procumbent position or to be- 
come approximated at the tips, suggesting that in Borhymia the reduction 
of the superior incisor formula has been accomplished by the suppression 
of the teeth homologous with the conical, procumbent, median incisors of 
Didelphys and Dasyurus. The upper canine is large, with swollen root 
and thick blunt crown. The premolars are closely crowded and increase 
rapidly in size posteriorly. All are double-rooted. The anterior pre- 
molar is in contact with the canine, and is placed transversely to the tooth 
row. The crown is slightly compressed laterally, and the heel rounded, 
without heel cusp. The median premolar is similar to the preceding 
tooth, but carries a larger heel. The posterior premolar is greatly 
enlarged and the heel broad, extending around the inner side of the 
crown. The anterior margin of the tooth is more or less abraded by con- 
tact with the similarly enlarged lower posterior premolar. The worn 
molars bear a superficial resemblance to the teeth of Sarcophitus , but 
their thylacyne structure is fully apparent, when it is remembered that in 
Sarcophilus the broadening of the upper molar cusps is produced by basal 
fusion with the outer row of styles, of which the antero-external alone is 
present in Boryhcena. The first, second and third molars increase regu- 
larly in size posteriorly. The protocone is small and ledge-like, disap- 
