4 1 2 PATAGONIAN EXPEDITIONS : PALAEONTOLOGY. 
former shows distinctly the unfused intercentrum, as in Borhymia and_ 
Am phipvovivevva. The third, fourth and fifth cervicals (PL LXII, fig. 8) 
are closely applied. The tops of the neural arches have been broken, but 
their bases are almost in contact, obliterating the intervertebral spaces. 
This may indicate the beginning of a fusion comparable to that in Di- 
delphys . The transverse processes resemble those of Didelphys , except 
that the anterior lamina of the transverse process of the third cervical is 
wanting. In this respect, Micvobiothevium resembles Dasyuvus. The 
bases of the transverse processes are pierced by a canal for the vertebral 
artery. These processes are imperfectly preserved on the side repre- 
sented in the figure. The articular surfaces of the centra are flat. A 
median ventral keel is indicated on the axis. A prominent tubercle is de- 
veloped on either side of the middle line on the posterior ventral border 
of the centrum of the fourth cervical. Corresponding swellings are faintly 
indicated on the lower borders of the centra of the third and fifth cer- 
vicals also. 
According to Ameghino (1894, p. 105), the feet were plantigrade and 
probably pentadactyl. 
Systematic Position and Affinities. — The affinities of Microbiothevium 
are unquestionably didelphid. Among living forms it approaches most 
closely some of the subspecies of Caluvomys laniger. The genus cannot 
be regarded as transitional to any of the living opossums, as the degree 
of reduction of the outer cingulum, styloid cusps and metacone spur in 
the upper molars is greater. It has been suggested (Bensley, 1903, p. 
208) that Microbiothevium , or some allied genus, is ancestral to the Cse- 
nolestidse and the Santa Cruz thylacynes. So far as the latter are con- 
cerned, the relationship suggested is still problematic, but it is extremely 
probable, as will be shown later, that the Csenolestidae have been derived 
from a didelphid ancestor. 
Microbiotherium tortor (Ameghino). 
(Plate LXII, Figs, i, 2, 2 a.) 
Hadvovhynchus tovtov Amegh. ; Nuevos Restos Mamif. Fos. Patagonia 
Austral, p. 25, Aug., 1891 ; Revista Argentina Hist. Nat. I, entr. 5a, 
p. 31 1, Oct., 1891. 
Hadvovhynchus towns Amegh. ; Ibid. 
This species is represented by an imperfect cranium and mandible (No. 
