SINCLAIR: MARSUPIALIA OF THE SANTA CRUZ BEDS. 



343 



FIG. 2. 



in that the number above and below is the same. In Amphiprwiverra 

 the median pair are conical and approximated at the tips, as in Dasynms 

 and Didelphys. According to Ameghino, the number of teeth having 

 deciduous predecessors is greater in the Santa Cruz forms than among 

 existing marsupials (see pp. 348, 378). Unfortu- 

 nately it has not been possible to check this impor- 

 tant observation by the material in the Princeton 

 collection. It would not be surprising if Mio- 

 cene marsupials retained some trace of the fuller 

 dental replacement characteristic of the placentals. 

 An extreme is reached in Thylacynus, where the 

 deciduous predecessors of the posterior premol- 

 ars are minute triangular plates displaced before 

 birth. In the Santa Cruz forms (cf. Cladosictis, 

 PI. LIX, fig. 6) replacement does not occur until 

 the animal is fairly mature. 



3. The atlanteal intercentrum is unfused with 

 the base of the neural arch in Borhycena and 

 Amphiprovi'verra, as it is also in Thylacynus 

 (text fig. 5, c\. In Frothy lacynus and Cladosictis 



1 . fnylacynus cynocephalus, 



complete fusion has taken place, with oblitera- rig h t scapula, x }. 

 tion of the sutures. An atlanteal foramen for the 



transmission of the spinal nerve and vertebral artery is present in all the 

 Santa Cruz genera, except Borhycena, which resembles Phascolomys 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 cer- 

 vicals are perforated for the transmission of the vertebral artery. The 

 dorso-lumbar vertebral formula was probably the same as in Thylacynus : 

 thirteen dorsals and six lumbars. As in that genus, the anticlinal verte- 

 bra is the tenth dorsal. Two vertebrae are coossified in the sacrum. The 

 tail was undoubtedly long, very heavy, and greatly thickened at the base. 



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

 shortening is especially noticeable in the bones of the fore arm and fore 

 leg, which are elongated in Thylacynus (text fig. I, b, of, e), an adaptation 

 to cursorial habits. In all, the radius and ulna are capable of some de- 

 gree of pronation and supination. The tibia and fibula are unfused. The 



