GLIRES OF THE SANTA CRUZ BEDS. 425 



as in that genus, has distal facets only for the meso- and ectocuneiforms ; 

 the ecto- and entocuneiforms are longer and narrower than in the latter. 

 The shape of the navicular and entocuneiform make it clear that at least 

 one large tibial sesamoid was present. 



The metatarsus consists of five members. Metatarsal I is extremely 

 slender and is relatively much more reduced than in Steiromys, not being 

 more than half as long as II ; it is, however, provided with a complete 

 distal trochlea and evidently carried phalanges. Metatarsals II, III and 

 IV are very slender and of nearly equal length, though they are so ar- 

 ranged as to make III project farthest distally, while IV is slightly shorter 

 and heavier than the others ; V is shorter than IV and has a well defined 

 projection from the fibular side of the head, as in Erethizon and Steiromys. 



The phalanges, so far as they are known, resemble those of the manus, 

 except for their greater length and slenderness. 



Relationships. It is evident that Sciamys (and no doubt the same is 

 true of the closely allied genus Acaremys} is related to Steiromys and 

 perhaps referable to the same family, yet with such differences as are at 

 least of subfamily rank. No existing genus can be derived from either of 

 the genera of the Acaremyince, which were probably terrestrial animals 

 that, in a measure, took the place of the rats and mice of the northern 

 hemisphere. It is not unlikely that the invasion of the murines, which 

 took place in the great migration from the north at the close of the Mio- 

 cene, may have been the direct or indirect cause of the extinction of these 

 small mouse-like and rat-like porcupines. 



The species of Sciamys have been distinguished almost entirely in 

 accordance with differences of size, but this is an untrustworthy criterion 

 and needs to be supplemented by other characteristics. 



SCIAMYS PRINCIPALIS Ameghino. 



(Plates LXVI, Figs. io-io 6 ; LXVII, 7-7".) 



Sciamys principals Amegh. ; Enumeracion sistem., etc. ; 1887, p. 9. 



Among the skulls preserved in the various collections, the one which 

 best agrees with the type of this species is characterized by its very narrow 

 and gently tapering rostrum, a feature strikingly different from all the 

 other skulls. As in most of the species of this genus, the incisors are very 

 narrow and slender ; in the mandible they have plane faces, but in the 

 upper jaw they are slightly convex. 



