j6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I49 



Middle cuneiform. — The dorsoventrally elongate middle cunei- 

 form is the smallest element of the Meniscotherium tarsus. Its 

 limited dorsal surface has convex proximal and distal margins that 

 lap slightly onto the dorsal surfaces of the navicular and second 

 metatarsal respectively. Both the proximal and distal surfaces of 

 the element are dorsoventrally concave, that for the navicular more 

 decidedly so. The surface for the second metatarsal is, moreover, 

 transversely convex and tapers ventrally. The medial and dorso- 

 ventrally compressed lateral surfaces of the middle cuneiform artic- 

 ulate in an uneven, arcuate facet with the internal cuneiform and in 

 a narrow dorsoventrally elongate facet with the external cuneiform 

 respectively. Behind the upturned ventral margin of the navicular 

 surface a small but distinctly rugose prominence, and possibly the 

 adjacent tip of the internal cuneiform, may well have received a part 

 of the tibialis posterior, which extending through the ventral notch of 

 the navicular would have been inserted as well on the ventral tubercle 

 of that bone, as mentioned in a foregoing section. This muscle, of 

 course, functions with the peroneus longus in extending the tarsus, 

 and against it in turning the foot inward. 



The Phenacodus middle cuneiform is much like that in Menisco- 

 therium, but the proximal and distal facets, though similar, are a 

 little more flattened, notably that for the navicular. This is true 

 also for the Tetraclaenodon middle cuneiform, and the dorsal surface 

 is more nearly square. The medial facet for the Phenacodus internal 

 cuneiform is also relatively flat and essentially straight, rather than 

 arcuate. It may be further noted that the ventral tubercle, evidently 

 for the tibialis posterior, though large in Phenacodus, is relatively 

 not so rugged as in Meniscotherium. This portion of the Tetraenodon 

 middle cuneiform is missing in the specimen examined. 



External cuneiform. — The external cuneiform is significantly larger 

 than the middle cuneiform in Meniscotherium, with its surface for the 

 third metatarsal more distal than the corresponding surface on the 

 middle cuneiform. The dorsal surface on some specimens appears 

 to be nearly twice as long proximodistally as that of the middle cunei- 

 form. The proximal surface for the navicular is dorsoventrally elon- 

 gate and ventrally tapering. That on the distal surface for the third 

 metatarsal is also dorsoventrally elongate but somewhat projecting 

 ventrally, and distinctly concave. It is also transversely broad ante- 

 riorly and markedly constricted about midway. The medial surface 

 shows a narrow, dorsoventrally elongate facet, adjacent to the proxi- 

 mal surface, for articulation with the middle cuneiform. Distal to 



