I9 o S .] SINCLAIR— FAUNA OF SANTA CRUZ BEDS. 77 



phys. The posterior premolar is preceded, in Cladosictis, by a 

 deciduous tooth resembling the first molar. x\ccording to Ame- 

 ghino, the median premolar and canine in this genus also have 

 deciduous predecessors, and in Borhycena the canine is said to dis- 

 place a deciduous tooth. 



3. The atlantal intercentrum is unfused with the base of the 

 neural arch in Borhycena and Amphiproviverra, as it is also in 

 Thylacynus. In Prothylacynus and Cladosictis complete fusion has 

 taken place with obliteration of the sutures. An atlantal fora- 

 men for the transmission of the spinal nerve and vertebral artery is 

 present in all the genera except Borhycena, which resembles Phasco- 

 lomys in transmitting the nerve and artery through a groove in the 

 anterior margin of the neural arch. The axis carries a large hatchet- 

 shaped neural spine. The bases of the transverse processes of the 

 second to the seventh cervicals are perforated for the transmission 

 of the vertebral artery. The dorso-lumbar vertebral formula was 

 probably nineteen as in Tiiylacynus : thirteen dorsals and six lum- 

 bars. As in that genus, the anticlinal vertebra is the tenth dorsal. 

 Two vertebras are coossified in the sacrum. The tail was undoubt- 

 edly long, very heavy and greatly thickened at the base. 



4. The limbs are short in proportion to the length of the body 

 and the feet small with spreading toes. The trochlear surface of the 

 astragalus is short and flat with feebly differentiated facets for the 

 tibia and fibula which latter articulates with the calcaneum. In 

 Prothylacynus, the hallux is reduced to a deformed metatarsal, 

 which carries no phalanges and terminates distally in a blunt rounded 

 knob. In Cladosictis, the hallux is small, judging from the size of 

 its articulation on the entocuneiform. It may have supported 

 phalanges. The hallux in Amphiproviverra is large and opposable 

 indicating that this genus was probably arboreal. The pollex is 

 known in Amphiproviverra and Cladosictis. In these genera, the 

 phalanges of the pollex are deflected toward the inner side of the 

 foot as a result of the enlargement of the outer condyle of the 

 metacarpal of the thumb. In gait, the Santa Cruz thylacynes 

 were probably plantigrade. In striking contrast with these extinct 

 genera the pes of Thylacynus shows a peculiar cursorial modification. 

 Not only is the gait of this animal digitigrade, and the hallux 

 entirely obliterated, but the ectocuneiform has shifted t6\vard the 

 outer side of the foot until it is supported almost entirely by the 



