486 PATAGONIAN EXPEDITIONS I PALEONTOLOGY. 



SCHISTOMYS ERRO Ameghino. 



(Plate LXVIII, Figs. 26-26".) 



Schistomys erro Amegh. ; Enumeracion sistem., etc., 1887, p. 13. 



This species, though the type of the genus, is much less common and 

 less well known than S. ellipticus, from which it is distinguished by its 

 very much smaller size. So far, no transitional forms, tending to connect 

 the two species, have been found and there can be little doubt that the 

 separation is justified. 



In the Princeton collection S. erro is represented by the crushed skull 

 of a young individual, with the milk premolars in place and m^ just erupt- 

 ing, but still entirely unworn. The specimen is too badly damaged for 

 any instructive comparison with the skull of 5. ellipticus, except to note its 

 greater relative breadth and the less triangular palate. 



Associated with the skull are the distal end of the tibia, the astragalus 

 and calcaneum ; these bones differ only in size from those of S. ellipticus. 



MEASUREMENTS. 



Upper dentition, length dp-mi 013 Palate, width at nr- oio 



DpA, length (i. e., antero-posterior diam- Tibia, distal width 006 



eter) 0025 " " thickness 006 



Dpi, width (j. e., transverse diameter) .0025 Astragalus, length 009 



Skull, interorbital width. 018 " width of trochlea 005 



Palate, width at dp 006 



PHANOMYS Ameghino. 



Phanomys Amegh. ; Enumeracion sistem., etc., 1887, p. 13. 



This interesting genus is unfortunately known only from a few grind- 

 ing teeth, which, though hypsodont, are conical and closed at the base, 

 where the valley between the two prisms becomes very shallow and dies 

 away. Evidently, Phanomys indicates the transition between the cavies 

 of the Patagonian beds, with rooted molars, and the typical Santa Cruz 

 forms with rootless teeth, open at the base. 



The two species named differ only in size. 



PHANOMYS MIXTUS Ameghino. 



Phanomys mixtus Amegh.; Enumeracion sistem. etc., 1887, p. 13. 

 Larger, the upper teeth measuring each 3-4 by 2-3 mm. 



