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 A mphiproviverra is perhaps nearest to the ancestral form in 
foot structure and shows least reduction in the heel of Mi. At the other 
extreme is Borhycena 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 
A mphiproviverra appears to have departed less in podial and dental 
structure than any of the others. 
DIDELPH YIDAi. 
MICROBIOTHERIUM Ameghino. 
(Plate LXII, Text-fig. 6.) 
Microbiotherium Amegh.; Enum. Sist. Especies Mamif. Fos. Patagonia 
Austral, pp. 6-7, 1887. 
Hadrorhynchus Amegh.; Nuevos Restos Mamif. Fos. Patagonia Austral, 
p. 25, Aug., 1891 ; Revista Argentina Hist. Nat., I, entr. 5*7, p. 31 1, 
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 Didelphyidse that the propriety of its ref- 
erence to the latter family seems beyond question. 
Dentition (PI. LXII, figs. 1-6). — The dental formula is f, 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. 1) 
retains in place the third incisor only. This tooth resembles the corre- 
sponding element in Didelphys. 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 
