LÖNNBERG, MAMMALS FROM ECUADOR AND RELATED FORMS. 29 



This correspondence between Stolzmann's and Söder- 

 ström^ notes, the similar name, and the origin of both ani- 

 mals from Ecuador made me at first believe that I had received 

 a specimen of Taczanowski's Paca. The length of the skull 

 of this specimen was only 2 mm less than the corresponding 

 measurement recorded by Stolzmann. But on the other 

 hand there are several very important differences as the follow- 

 ing comparison proves. The most striking of these is perhaps 

 that the greatest width of this skull is only 76,5 mm, while 

 the same dimension of Stolzmann's type is 90 mm. The 

 shape of the zygomatic arches is also very different. In the 

 type of Taczanowski's Paca they bulge out very much so that 

 the contour line is strongly convex when the skull is seen from 

 above, or from below. In the present specimen the zygomatic 

 arches do not bulge out at all, but the contour line is practi- 

 cally quite straight and evenly sloping forward to the snout 

 from the level of the greatest breadth behind. The outer 

 surface of the zygomatic arches is in the present specimen 

 almost perfectly smooth, not deeply pitted as in taczanowskii, 

 nor is the broadened portion of the zygomatic arches curved 

 inwards with their lower parts as in the latter, but their lower 

 edges are perfectly vertical. The zygomatic arches are de- 

 cidedly less developed in this alpine Paca than in A. tacza- 

 noivskii, which the following measurement s pro ve. 



Least width of the arch separating the orbit from 



foramen infraorbitale 3,3 mm 



but in taczanoivskii 5 » 



Height of zygomatic arch (below orbit) 31 » 



but in taczanoivskii 50 » 



This difference with regard to the development of the 

 zygomatic arches is very striking and evidently of impor- 

 tance considering the facts: 



l:o that this specimen is a fully adult male with even 

 the hindmost molars well worn, and 



2:o that at least in the typical Agouti [Coelogenys] paca 

 of Brazil the zygomatic arches are practically fully deve- 

 loped, curved inwards with their lower margin, and pitted 

 on their exteriör surface already at the time when the hind- 

 most molar is cutting the gum, and before it is worn a t all. 



