SCOTT : LITOPTERNA OF THE SANTA CRUZ BEDS. 3 



tyla. The upper molars have two equal, concave and somewhat crescentic, 

 external lobes, which unite inamesostyle, and two principal internal, more 

 or less conical cusps (the pro to- and hypocone). Anterior and posterior 

 conules are prominently developed, but do not fuse into well-defined trans- 

 verse crests, the posterior crest being especially incomplete. The internal 

 cingulum is so elevated as to enclose deep fossettes, the number and position 

 of which differ in the various genera. The internal cusps are always close 

 together and often connate, or connected by a ridge. The lower molars 

 are composed of two crescents, one behind the other, a pattern which is 

 common in early members of both Artiodactyla and Perissodactyla, such 

 as the Anoplotheres, Palasotheres, Titanotheres, etc. A very character- 

 istic feature is the pillar or spur in the inner concavity of the posterior 

 crescent, which, there is reason to believe, was originally common to all 

 of the Litopterna, but in the Santa Cruz several genera have it in very 

 reduced form and others have lost it altogether. It is more reduced in 

 the Proterotheriidae and in Adianthus, more developed in the other Mac- 

 rauchenidae. The Pampean genus Macrauchenia has no trace of it in the 

 true molars, but retains it in the milk-premolars. The third lower molar 

 may have a small talon, but more commonly there is none. 



All of the antemolars have predecessors in the milk-series, the teeth 

 of which in general resemble their permanent successors. The deciduous 

 grinding teeth, except dp 4, unlike those of the perissodactyls, are simpler 

 than the permanent molars. 



The skull differs much in the two families, being of more normal ap- 

 pearance in the Proterotheriidae than in the Macrauchenidas, in which the 

 nasal bones are greatly shortened and the anterior nares correspondingly 

 enlarged, showing that a proboscis was at least in an incipient stage of 

 development. No alisphenoid canal is present in any known member of 

 the group and the foramina rotundum and ovale are confluent. The 

 tympanic is very loosely attached to the skull and imperfectly ossified 

 not forming a bulla, but merely a small plate, with large, irregular meatus 

 externus, which is not at all tubular. A high sagittal crest is present in all 

 of the genera of both families. 



One of the most noteworthy differences between the two families is in 

 the length of the neck ; in the Proterotheriidae the neck is very short and 

 the canal for the vertebral artery occupies the normal position, perforating 

 the transverse processes of the cervical vertebrae. In the Macrauchenidae, 



