igos .] SINCLAIR— FAUNA OE SANTA CRUZ BEDS. 79 



link between the polyprotodont and diprotodont marsupial sub- 

 orders in possessing, in the lower jaw, the tuberculo-sectorial type 

 of molar characteristic of the polyprotodonts combined with a 

 diprotodont modification of the median incisors. One of the 

 minute Santa Cruz forms has the same inferior dental formula as the 

 opossums. Unfortunately, nothing is known of the upper denti- 

 tion, skull and feet of this important transitional form. 



The second subfamily, the Palseothentinae, contains the largest 

 of the Santa Cruz diprotodonts. The upper molars of the Palaeo- 

 thentinae resemble closely those of certain bunodont phalangers. 

 The first is fully quadritubercular. The second has a rudimentary 

 hypocone. The third and fourth are tritubercular. The lower 

 molars are lophodont. The posterior upper premolar and first 

 lower molar are modified as sectorial teeth. The dental formula 

 varies in the different genera but there are always four molars above 

 and below. The members of this subfamily form a regular pro- 

 gressive series in the shortening of the anterior portion of the 

 mandible and the reduction of the posterior lower premolar from a 

 double-rooted fully functional tooth to a single-rooted more or less 

 vestigial condition. 



The Abderitinse, the third subfamily, are the most specialized of 

 the Santa Cruz diprotodonts. The first lower molar is greatly en- 

 larged, vertically grooved, and notched along the cutting edge of 

 the crown, resembling in some respects the peculiar sectorial teeth 

 of the multituberculate Plagiaulacidae. The sectorial in Abderites, 

 however, possesses a large bicuspidate heel, which is lacking in the 

 Plagiaulacidae, and the remaining molars are quadritubercular. 



The Csenolestidae are examples of the restrictive influence of 

 competition on adaptive radiation. During the Santa Cruz epoch 

 they were crowded into obscurity by a horde of placentals, sloths, 

 rodents, and ungulates, and had no opportunity to attain the high 

 degree of adaptive specialization shown by the Australian dipro- 

 todonts, although so far as can be judged, they possessed as much 

 latent capacity toward variation as do their nearest living allies, the 

 phalangers. 



RELATIONSHIPS OF THE SANTA CRUZ MARSUPIALS. 



The Patagonian thylacynes do not represent the main line of 

 descent which ended in Thylacynus. In all the Santa Cruz genera 



