408 PATAGONIAN EXPEDITIONS: PALAEONTOLOGY. 



thylacynes, it is surprising to notice the extent to which they have 

 responded to adaptive specialization. No one of them is ancestral to the 

 others, but Ampliiproviverm is perhaps nearest to the ancestral form in 

 foot structure and shows least reduction in the heel of MT. At the other 

 extreme is Borhycsna which, so far as dentition goes, is a decidedly spe- 

 cialized animal. All the Santa Cruz genera are, apparently, divergent 

 branches of a common pre-Santa Cruz ancestral stock, from which 

 Amphiproviverra appears to have departed less in podial and dental 

 structure than any of the others. 



DIDELPHYID^E. 

 MICROBIOTHERIUM Ameghino. 



(Plate LXII, Text-fig. 6.) 



Microbiotheriiim Amegh.; Enum. Sist. Especies Mamif. F6s. Patagonia 



Austral, pp. 6-7, 1887. 

 Hadrorkynckus Amegh.; Nuevos Restos Mamif. Fos. Patagonia Austral, 



p. 25, Aug., 1891 ; Revista Argentina Hist. Nat, I, entr. $a, p. 311, 



Oct., 1891. 



Minute polyprotodonts, comparable in size to some of the smaller 

 South American opossums. Although placed by Ameghino in a separate 

 family, the Microbiotheridae, this genus possesses so many important 

 characters in common with the Didelphyidae that the propriety of its ref- 

 erence to the latter family seems beyond question. 



Dentition (PI. LXII, figs. 1-6). The dental formula is I, i, f, I, as in 

 Didelphys. The upper incisors are unspaced and the lateral tooth is 

 separated from the canine by a long diastema. The skull of Microbio- 

 therium tortor in the Princeton collection (No. 15,698, PI. LXII, fig. i) 

 retains in place the third incisor only. This tooth resembles the corre- 

 sponding element in Didelpliys. As in that genus, the median incisors 

 were probably procumbent and approximated at the tips, judging from the 

 inclination of the alveoli, but this portion of the premaxillae has been 

 somewhat crushed and the inference cannot be fully verified. The upper 

 canine is a robust tooth, rather blunt, with the crown but little recurved. 

 The premolars are three in number and closely crowded. The anterior 

 premolar is single-rooted, the median and posterior double-rooted. The 

 latter is the largest of the series. A photograph of a specimen in the La 



