SINCLAIR: MARSUPIALIA OF THE SANTA CRUZ BEDS. 443 



molar of Abderites and the notched and fluted sectorial teeth of the 

 plagiaulacid Multituberculata. This resemblance is confined to the an- 

 terior half of the sectorial, the posterior portion in Abderites supporting a 

 large bicuspidate heel. The posterior molars show no indication of cusp 

 reduplication and still retain the three cusps of the trigonid, indicating 

 their tuberculo-sectorial origin. In Hypsiprymnodon and Bettongia the 

 posterior premolars closely resemble the sectorials of the Multitubercu- 

 lata. The Bettongiinae and Abderitinae illustrate a case of convergence, 

 where much the same form has been assumed by totally different teeth. 

 The homologies of the sectorials of the Plagiaulacidae are uncertain, but 

 they have been interpreted as posterior premolars, while in the Abderitinae 

 the sectorial is unquestionably the first molar. 



In a former paper the writer (1905, p. 81) stated that: "the Caenoles- 

 tidae resemble the primitive phalangers in so many respects that it is im- 

 possible to escape the conclusion, that the two famlies are related and not 

 merely convergent groups. With the exception of Halmarhiphtts, a 

 persistent ancestral type, the Santa Cruz diprotodonts possess specializa- 

 tions in dental structure which prevent their being regarded as direct 

 ancestors 'of the phalangers, but favor the idea that both groups are 

 descended from a common ancestry." While substantially the same con- 

 clusions are still held, it is proper to point out the evidence in favor of 

 the view that the striking similarity in dental structure displayed by the 

 two families may be explained by convergence. So far as the foot struc- 

 ture of the Caenolestidae is known (p. 418), the pes shows no trace of 

 syndactyly, while in all the phalangers it is syndactylous. A further dif- 

 ference appears in the development of sectorial teeth, which, when present 

 in the Caenolestidae, are confined to the posterior superior premolar and 

 first lower molar, while in the phalangers the sectorial function may be 

 transferred to the posterior inferior premolar in some of the higher forms 

 ( Trichosums, Phalanger}. Until the upper dentition, skull and feet of the 

 Caenolestidae, and especially of the primitive members of the family, are 

 fully known, this must remain an unsettled question. At present the ar- 

 guments in favor of the alternatives expressed are about equally balanced. 



The Caenolestidae lend no support to the latest classification proposed 

 for them by Ameghino (1903, pp. 153-159) in which they are grouped as 

 the suborder Paucituberculata, order Plagiaulacoidea, superorder Dipro- 

 todonta, the latter including also the orders Hypsiprimnoidea and Ro- 



