32 
PATAGONIAN EXPEDITIONS : PALAEONTOLOGY. 
as occurs in a free state in many of the hystricomorph rodents, with 
which it coincides in position. The proximal surface of the cuboid is 
strongly convex in dorso-plantar section and broadly concave transversely. 
Internally, it is in contact with the navicular by a large oval facet concave 
in dorso-plantar section and convex proximo-distally, below which are 
two oval facets, with their major axes at right angles, for contact with the 
outer cuneiform. The facet for the fourth and fifth metatarsals is a large, 
irregularly oval area, concave in all dimensions and not differentiated 
into separate articular surfaces for these two elements. The peroneal 
groove is deep and the overhanging tubercle very large. Of the cunei- 
form series, the outer is the largest. It is irregularly quadrilateral in out- 
line dorsally, decreasing in transverse diameter toward the plantar surface, 
where it supports a long process terminating in a blunt tubercle. The 
proximal surface is semicircular in outline and almost plane. Of the two 
oval facets for the cuboid, the dorsal is convex in proximo-distal section 
and almost a plane surface at right angles to this, while the plantar sur- 
face is plane. Internally, there is a narrow, almost plane surface for the 
mesocuneiform proximally and two semicircular, approximately plane 
facets distally for the second metatarsal. The surface for the third meta- 
tarsal is reniform, wider dorsally than at the plantar margin, convex from 
side to side and concave in dorso-plantar section. The mesocuneiform is 
quite small. Its proximal surface is semicircular and almost plane. 
Externally and proximally, there is a narrow, plane surface for the outer 
cuneiform. Distally, the facet for the second metatarsal is sigmoid in 
dorso-plantar section and convex transversely. Internally and proximally, 
there is a single facet, variable in form, for the internal cuneiform. The 
latter is scale-like, terminating distally in a rugose tubercle without any 
trace of an articulation for the hallux. The third and fourth metatarsals 
are of approximately the same length and weight, while the second and 
fifth are much shorter and more slender, the fifth slightly exceeding the 
second in length. Proximally, the metatarsals interlock with each other 
and with the tarsus. The proximal surfaces are sufficiently well shown in 
the figure (PI. V, fig. io) to dispense with further description. The shafts 
are slightly flattened. Well developed keels are present distally. Except 
for their superior size, the phalanges are indistinguishable from those of 
the manus. The terminal phalanges are laterally compressed hoofs, with- 
out clefts. 
