78 SINCLAIR— FAUNA OF SANTA CRUZ BEDS. 



L April 13. 



cuboid. In the Santa Cruz forms, this shifting has progressed to 

 about the same extent as in Sarcophilus. There is no trace of syn- 

 dactyly. The manus and pes are pentadactyl in Amphiproviverra 

 and Cladosictis. The manus is pentadactyl in Borhycena and prob- 

 ably also in Frothy lacynus. The hallux is reduced to a vestige in 

 the latter genus. Its condition in Borhycena is unknown. 



5. The pelvis is without trace of epipubic ossifications in Clado- 

 sictis. The pubes are not preserved in the only specimen of Pro- 

 thylacynus in the Princeton collection, and the pelvis of Bor- 

 hyana and Amphiproviverra is unknown. The patella is ossified in 

 Amphiproviverra and Prothylacynus. The radius and ulna are 

 capable of some degree of pronation and supination. The tibia and 

 fibula are unfused. The inner humeral epicondyle is perforated by 

 a large foramen in Prothylacynus and Cladosictis ; imperforate in 

 Amphiproviverra. The supinator ridge terminates in a hook-shaped 

 extremity in Prothylacynus. This is wanting in Amphiproviverra 

 and Cladosictis. 



THE DIDELPHYID.E. 



The Didelphyidas are represented in the Santa Cruz fauna by 

 several genera of which Microbiotherium is the best known. In 

 dental formula and the structure of the lower molars Microbio- 

 therium agrees with Didelphys, differing from all the opossums in 

 the greater reduction of the outer cingulum, styloid cusps, and 

 metacone spur in the upper molars. The posterior premolar is en- 

 larged in both the upper and lower dental series. The premolars 

 are double-rooted in the majority of the species and decrease in 

 size anteriorly. The molars in both series decrease in size pos- 

 teriorly as in the existing didelphyd genus Caluromys. 



THE CVENOLESTID.E. 



This family, better known as the Epanorthidae, includes all the 

 Santa Cruz diprotodont marsupials. As the genus Palceothentes, 

 defined by Ameghino in 1887, has priority over Epanorthus, pro- 

 posed by him two years later, necessitating the rejection of the 

 latter, the family has been renamed after its best known representa- 

 tive, Ccenolestes. All the members of this family are small animals 

 and are very incompletely known. 



Three subfamilies may be recognized. The more primitive 

 members of the first of these, the Caenolestinae, form a connecting 



