Eaton — Vertebrate Fossils from Ayusbamba, Peru. 153 



search was made along the bottom and sides of a small ravine 

 where this specimen occurred, but the rest of the skull was 

 not found. 



There was, in the possession of Dr. Romualdo Aguilar of 

 Cuzco, a fragment of a small equine skull with several teeth, 

 that came from very near this same place. Although I have had 

 no opportunity to examine this specimen closely, its general 

 appearance and the circumstances under which it was collected 

 lead me to believe that it may be from the same skull as the 

 portion taken later by the Yale Expedition. I trust that Dr. 

 Aguilar will soon publish views and a description of his specimen. 



The, posterior part of the skull is shown in Plate YI, figure 

 1, together with two disconnected but important pieces of the 

 nasal and premaxillary bones that are placed, as nearly as 

 possible, in their true relative positions. Compared with the view 

 of the skull of Onohippidium compressidens (fig. 2), it is at 

 once seen that the skull from Ayusbamba was characterized by 

 slender, freely projecting nasal bones, very similar to those 

 of Onohippidium and Hippidium bonaerense. A conspicuous 

 cheek-groove, beneath the elongated ascending process of the 

 premaxilla, corresponds in part to the anterior chamber of the 

 "fossa lacrymalis" of Onohippidiu?)} . It is the latter genus 

 that this skull approaches most nearly in the form of the nasal 

 bones, in the cheek-grooves of the maxillae, and in the depth of 

 the alveolar portions of the maxillae below the facial crests. In 

 the shape of the zygomata and in the narrower preorbital breadth 

 of the skull, as seen in Plate YII, figure 1, the resemblance is 

 rather with Hippidium bonaerense. The posterior view of the 

 skull is shown in Plate YI, figure 3, and its palatal or lower 

 surface in Plate YII, figure 2. The skull, while indicated by the 

 sutures to be that of a full-grown animal, is smaller in nearly 

 every dimension than that of Hipjndiumbonaerense, especially 

 in the transverse measurements. The contrast in size with the 

 skull of Onohippidium is even greater. To compare the skull 

 from Ayusbamba with those of Onohip>pidium compressidens 

 and Hippidium bonaerense as fully as possible, I have made use 

 of valuable data recorded by Sef ve, and have arranged some of 

 the principal measurements of these three crania in the accom- 

 panying table. 



Although the skull from Ayusbamba and that of Onohippi- 

 dium compressidens differ greatly in actual size, their proportions 

 are so nearly alike that their generic separation by cranial charac- 

 ters alone, irrespective of size, would be difficult and unsatisfac- 

 tory. In referring the Ayusbamba skull provisionally to Para- 

 hipparion I am influenced not only by the slight differences in 

 cranial form, but also by what is known of the geographic 

 distribution of the genera Parahipparion and Onohippidium. 



