heavy heads, short limbs with usually five semidigitigrade toes. The genera are mostly distinguished by the relative development of the protocone on the upper molars and the talonid on the lower ones. Figure 138 gives a typical marsupial upper molar 2 and a lower molar 2 to show the sense in which these terms are used. The Santa Cruz genera are the best known and I therefore use them as a basis for comparison with the less known Deseado forms, of which we found but the one genus _Pharsophorus_ at all abundant. In addition to this, Ameghino has reported a gigantic form designated _Proborhyaena_. The following table indicates the relationships of the best known genera. [Footnote 24: There is a discussion as to the homologies of the premolars of marsupials and placental mammals, the one proposition being that marsupials have three premolars and four molars, the other that they have four premolars and three molars as in placentals. The evidence is not conclusive as to either proposition, but in this paper I have designated these teeth along the latter line of thought.] ========================================================================== | | | | |Talonid on| | Age |Formula|Protocone| Upper | Lower |Symphysis | | | | Molar | Molar | | | | | 3 | 3 | --------------+-------+-------+---------+---------+----------+------------ Cladosictis |Santa |4 1 4 3|on pm. |Protocone|Small |Ligamentous | Cruz |-------| 4-m. 3 |Paracone |basin | | |3 1 4 3| |vestigal |with one | | | | |Metacone |post. cusp| | | | |Ant. ext.| | | | | | style | | --------------+-------+-------+---------+---------+----------+------------ Amphiprovivera|Santa |4 1 4 3|on pm. |Protocone|Basin with|Ligamentous | Cruz |-------| 4-m. 3 |Paracone |two post. | | |3 1 4 3| |Ant. ext.| cusps | | | | | style | | --------------+-------+-------+---------+---------+----------+------------ Prothylacynus |Santa |4 1 4 3|on pm. |Protocone|Small |Fused | Cruz |-------| 4-m. 1 |vestigal |basin | | |3 1 4 3| |Metacone |with one | | | | | |post. cusp| --------------+-------+-------+---------+---------+----------+------------ Borhyaena |Santa |3 1 4 3|vestigal |Paracone |No basin |Fused | Cruz |-------| |Ant. ext.|One post. | | |3 1 4 3| | style | cusp |