SCOTT : LITOPTERNA OF THE SANTA CRUZ BEDS. I I9 



the horse ; this is long, slender, much compressed laterally and tapering 

 distally ; the proximal end is expanded in the antero-posterior direction, 

 forming a large, thin, bony plate. Though chiefly posterior, this expansion 

 is less exclusively so than in the horse. 



The skull oi Macrauchenia fatachonica is manifestly of the same type as 

 that of Theosodon, but has undergone extensive modifications, chiefly in 

 response to the development of the proboscis. The face has become rela- 

 tively more elongate, owing to the shifting of the orbits, which are now 

 behind the dental series and are completely encircled in bone. In corre- 

 lation with this shifting of the orbits, the zygomatic arches are shortened 

 and the temporal openings reduced in size. The cranium is shorter and 

 more rounded and the sagittal crest is replaced by a broad sagittal area. 

 The premaxillas are shorter and broader and are thoroughly fused together 

 in the median line, as are also the maxillaries for the greater part of their 

 length, thus forming a long, solid rostrum. The anterior narial opening 

 is greatly reduced in length and shifted backward between the orbits and 

 vertically above the posterior nares. Behind the superior narial opening, 

 there is a deep depression of the frontals, at the bottom of which are con- 

 spicuous pits and ridges for the attachment of the muscles of the proboscis. 



As Ameghino has demonstrated ('94*5, 277 ff ) several genera of the 

 family, of intermediate geological dates, form a series of gradations in 

 dentition and skull structure between Theosodon and Macrauchenia. Of 

 these the most important are Mesorhinus and Scalabrinithermm. The 

 . former, unfortunately still very incompletely known, has the premaxillaries 

 ankylosed and part of the maxillaries also ; behind this symphysis is a 

 groove, which widens and deepens posteriorly and leads back to the ante- 

 rior nasal opening, which begins much farther back than in Theosodon and 

 not so far as in Scalabrinithermm. In the latter genus the approximation 

 to Macrauchenia is still more marked ; the orbits have shifted backward, 

 bringing the anterior rim over m- ; the nasals are much reduced, though 

 still considerably longer than in the Pampean genus. The anterior nares 

 have been displaced backward, though not so far as in Macrauchenia, and 

 there is as yet no depression of the frontals for muscular attachments. The 

 upper molars have a somewhat more complex pattern than in Theosodon, 

 a change which principally consists in the deepening of the external 

 crescents and in the development of the antero-internal cingulum into a 

 more independent crest. 



