4l8 PATAGONIAN EXPEDITIONS: PALEONTOLOGY. 



genera. It is interesting to note that the migrant, Erethizon, which is on 

 the outskirts of the distributional area, has undergone less modification 

 than Coendou, which has remained in the original home of the group. 



STEIROMYS DETENTUS Ameghino. 



(Plate LXVI, Figs. 6-7".) 



Steiromys detent^ts Amegh. ; Enumeracion sistem., etc. ; 1887, p. 9. 



This species is distinguished, not only by its smaller size, which is very 

 constant in many specimens, but also by the symmetrical shape of m , 

 which is smaller than m T or ^. In the table, the measurements of the 

 upper teeth and skull are from a specimen (No. 9,555) belonging to the 

 American Museum, those of the lower teeth and mandible from No. 15,- 

 094 in the Princeton collection. 



MEASUREMENTS. 



Upper dentition, length 044 Rostrum, width at anterior end 016 



Upper dentition, length p 4 m^ OI 9S Lower dentition, length p-j m-j- 021 



Upper dentition, length ml-m& 014 Lower dentition, length m m^ 015 



Upper incisor, width 004 Lower incisor width 004 



" " thickness 004 " " thickness 004 



Skull, estimated length 085 Mandible, depth below p ? 014 



STEIROMYS INTERMEDIUS sp. nov. 



(Plate LXVI, Figs. 3-4.) 



In size this species is intermediate between S. detentus and S. duplicatus, 

 though it cannot be called transitional, because of the constancy of those 

 two species in regard to size. However, size is not the only distinguish- 

 ing character ; the incisors are actually as large as in S. duplicatus and 

 are therefore relatively much larger ; in addition to the external raised 

 border or pillar, is a similar, though much less distinct, pillar on the 

 mesial border ; between the two the anterior face is depressed and flat. 

 The grinding teeth are rather small in proportion to the size of the jaw ; 

 nij is reduced in size and asymmetrical, the posterior lobe of the tooth 

 being much smaller than the anterior lobe. 



The type of the species is a mandible, No. 15,367, in the Princeton 

 Museum, and it is probable that the upper teeth from the Ameghino col- 

 lection, shown at Plate LXVI, fig. 3, should also be referred to the same 

 species. 



