412 PATAGONIAN EXPEDITIONS I PAL/EONTOLOGY. 



former shows distinctly the unfused intercentrum, as in Borhycena and 

 Amphiproviverra. The third, fourth and fifth cervicals (PI. 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, Microbiotherimn resembles Dasyurus. 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 Microbiotherium 

 are unquestionably didelphid. Among living forms it approaches most 

 closely some of the subspecies of Caluromys 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 Microbiotherium, or some allied genus, is ancestral to the Cas- 

 nolestidae 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 Caenolestidae have been derived 

 from a didelphid ancestor. 



MICROBIOTHERIUM TORTOR (Ameghino). 



(Plate LXII, Figs, i, 2, 20.) 



Hadrorhynchus tortor Amegh. ; Nuevos Restos Mamif. Fos. Patagonia 

 Austral, p. 25, Aug., 1891 ; Revista Argentina Hist. Nat. I, entr. 5^, 

 p. 311, Oct., 1891. 



Hadrorhynchus torvus Amegh. ; Ibid. 



This species is represented by an imperfect cranium and mandible (No. 



