SINCLAIR : MARSUPIALIA OF THE SANTA CRUZ BEDS. 
343 
in that the number above and below is the same. In Aniphiproviverra 
the median pair are conical and approximated at the tips, as in Dasyurus 
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 Thy lacy mis, 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 
Amphiproviverra, as it is also in Thy lacy tins 
(text fig. 5, c). In Prothylacynus and Cladosictis 
complete fusion has taken place, with oblitera- 
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 Thylacynns : 
thirteen dorsals and six lumbars. As in that genus, the anticlinal verte- 
bra is the tenth dorsal. Two vertebras 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 Thylacynns (text fig. 1, b , d, 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 
Fig. 2. 
right scapula, x 
