438 PATAGONIAN EXPEDITIONS: PALEONTOLOGY. 



nally. The tibia also is relatively much shorter and more slender than in 

 yiscaccia, in which the tibia is nearly twice as long as the radius, while in 

 Perimys it is only about one third longer ; the proximal portion is thick, 

 owing to the development of the cnemial crest, but for most of its length, 

 the shft is aslender and subcylindrical. 



The calcaneum has a short, straight and heavy tuber, with parallel dor- 

 sal and ventral borders, and an elongate distal portion, ending in an ob- 

 lique, concave facet for the cuboid. The trochlea of the astragalus is 

 narrow and deeply grooved and the relatively long neck has but little 

 inclination to the tibial side; the head is rather thick planto-dorsally. 

 The number of digits in the pes is not known, but it was quite certainly 

 not less than four and may have been five. The metatarsals are consid- 

 erably longer than the metacarpals, but they are very slender and evidently 

 the hind foot was shorter and far more slender than in yiscaccia. 



Relationships. Imperfect as are the known specimens of Perimys, they 

 suffice to prove the close relationship of the genus to yiscaccia and the 

 propriety of including it in the Chine hillidce. Not improbably it is but 

 little removed from the common ancestor of all the existing genera of that 

 family, for the genus is known from formations older than the Santa Cruz. 

 On the other hand, the Santa Cruz species are without phylogenetic sig- 

 nificance and represent a branch of the family which, though flourishing 

 at that period, soon became extinct and did not persist to the later epochs. 



The numerous species into which Ameghino has divided Perimys re- 

 quire revision, but the material is still too incomplete for any satisfactory 

 performance of this task. The Princeton and New York collections con- 

 tain several individuals which, though approximating Ameghino's types, 

 do not agree at all closely with them ; it will therefore be necessary either 

 to regard most of the species as variable groups, or else to increase their 

 number very considerably. 



Disregarding size, the species of Perimys fall naturally into two series, 

 though transitions between them occur, distinguished by the character 

 of p^ ; in one series p s is not very oblique, its postero-internal portion is 

 not very strongly compressed, and the valley opens externally ; in the 

 other series p t has a very oblique position, so that the valley opens an- 

 teriorly rather than laterally, and the postero-internal end is very narrow 

 and compressed. As the mandibular dentition of several species is not 

 known, this distinction cannot be made in a consistent manner. 



