GLIRES OF THE SANTA CRUZ BEDS. 453 



is small, hemispherical and set upon a slender neck ; the great trochanter 

 has lost its epiphysis in all the specimens, but was evidently very large, 

 and the digital fossa is very deep ; the second trochanter is prominent, 

 but less massive than in the modern genus, and, as in the latter there is 

 no trace of a third trochanter. The shaft is much stouter than in Perimys 

 and has not such a cylindrical shape, but is antero-posteriorly compressed ; 

 a strong linea aspera externa runs down the shaft for a greater or less dis- 

 tance from the great trochanter. The condyles project more prominently 

 behind the plane of the shaft than in Viscaccia and the rotular groove is 

 more symmetrical, the external border being raised as high as the internal. 



There is no way of determining the length of the tibia from the frag- 

 ments, but it clearly was very much longer and heavier than in Perimys, 

 more slender than in 1/iscaccia and it differs in several respects from the 

 latter; the external condyle has no such hook-like process from its 

 postero-external angle, the cnemial crest is decidedly less prominent and 

 the proximal part of the shaft is more deeply channelled on the posterior 

 side. The distal end is very much as in Viscaccia; the astragalar surface 

 is asymmetrical, with much larger facet for the external than for the 

 internal condyle ; the intercondylar ridge is low, but the posterior, tongue- 

 like process is both long and broad ; the internal malleolus, which is 

 quite prominent, is not grooved by a tendinal sulcus, as it so conspicu- 

 ously is in Perimys. 



Relationships. While Prolagostomus is still far too imperfectly known 

 for any definite determination of its phylogenetic significance, yet all that 

 we do know concerning it confirms Ameghino's original suggestion 

 ('87*, 12) that it is the actual and direct ancestor of Viscaccia. On the 

 other hand, it has certain unmistakable likenesses to Perimys, which indi- 

 cate that some early, pre-Santacruzian genus, not very far removed from 

 the latter, was the common ancestor of the family. 



I have seen no example of the supposed genus Sphceromys, but Ame- 

 ghino's description of it strongly suggests that it was founded upon a 

 young individual of the present genus, with dp- still in place. 



PROLAGOSTOMUS PUSILLUS Ameghino. 



(Plate LXVIII, Fig. 17.) 



Prolagostomus pusillus Amegh. ; Enumeracion sistem., etc.; 1887, p. n. 

 fSph&ramys irruptus Amegh. ; Ibid., p. 13. 



