GLIRES OF THE SANTA CRUZ BEDS. 417 



The entocuneiform is a large bone, with principal diameter proximo- 

 distal, but it is also very thick planto-dorsally ; the proximal facet for the 

 first sesamoid is nearly plane and the distal one for the first metatarsal is 

 saddle-shaped, while those for the navicular and second metatarsal are on 

 the fibular side. The mesocuneiform is very small and the ectocuneiform 

 is very large, both having nearly the same shape and proportions as in 

 Erethizon. The cuboid also is very much as in the latter, but has a some- 

 what more oblique facet for the calcaneum and does not come into contact 

 with the astragalus. 



The metatarsus is remarkably elongate, exceeding the metacarpus in 

 length much more than is the case in Erethizon. Metatarsal I is the 

 shortest of the series, but is stout and well developed ; the proximal end 

 is broad and thick and bears a large, saddle-shaped facet for the entocu- 

 neiform. The shape of this facet and the divergent position of the whole 

 hallux suggest that this digit was opposable, at least to a limited extent. 

 The shaft is broad and antero-posteriorly compressed, and the distal 

 trochlea, which is complete and has a low carina, is asymmetrical, being 

 displaced toward the fibular side. 



The remaining metatarsals differ little from one another in length, IV 

 being slightly the longest and II and V almost equal ; II, III and IV have 

 slender shafts, of transversely oval section, and arched forward ; the distal 

 trochlea is hemispherical, with a small, but well defined, supratrochlear fossa 

 above it, and a short, low carina on the plantar side. Metatarsal V has 

 an expansion of the proximal end, which ends in a blunt projection from 

 the fibular side, much as in Erethizon, but less heavy and prominent. 



The phalanges differ from those of the manus only in being longer and 

 heavier, the entire pes much exceeding the manus in all dimensions. 



Relationships. No reason is apparent, why we should not regard 

 Steiromys as the actual and direct ancestor of Erethizon, an animal which 

 is demonstrably of South American origin. The Santa Cruz genus is but 

 little more primitive than the recent one and the change from one to the 

 other involves but a slight modification. On the other hand, Steiromys 

 may also be an ancestor of Coendou, as is suggested especially by the 

 presence of the two sesamoids in the tarsus. So long, however, as we 

 remain in complete ignorance of the character of the tail in the fossil, we 

 can hardly decide this question. At all events, Steiromys must be near 

 to the common ancestor of both the North and South American recent 



