SCOTT : LITOPTERNA OF THE SANTA CRUZ BEDS. II 



A highly characteristic feature of the Proterotheriidae, which strongly 

 distinguishes them from the Macrauchenidse, is the structure of the feet. 

 These are moderately elongate, except in Thoatheriitm, which has very 

 long feet, and the elongation is largely due to the length of the phalanges, 

 the metapodials being quite short. In both manus and pes the third 

 digit is greatly enlarged and carries the whole weight of the body, while 

 the second and fourth are very much reduced and either form dew-claws, 

 or, in the case of the monodactyl Thoatheriitm, nothing remains of the 

 lateral digits except vestiges of the proximal ends of the metapodials. 

 This is a more complete degree of monodactylism than in any other known 

 mammal. Thus, in all of the Santa Cruz genera of the family, the aspect 

 of the feet is strikingly and surprisingly horse-like, a resemblance which is 

 borne out by the very equine character of the ungual phalanges. 



Despite this high degree of specialization, carpus and tarsus retain a 

 very primitive structure, which is not only not in the least equine, but is 

 not even perissodactyl. The carpus is almost taxeopod, except that the 

 scaphoid has extended over upon the magnum, the lunar having no 

 contact with the unciform, from which it is excluded by the articulation, 

 entirely lateral, between the magnum and pyramidal. The tarsus is 

 equally primitive ; the calcaneum has a large facet for the fibula and the 

 astragalus is widely separated from the cuboid, articulating with the navic- 

 ular by means of a convex head, which is less flattened than in the Macrau- 

 chenidse. The method of digital reduction, which in all known perisso- 

 dactyls, both fossil and recent, is according to the mode which Kowalevsky 

 called "adaptive," is in the Proterotheriidse "inadaptive." That is to say, 

 the vestigial digits retain the carpal and tarsal connections which they 

 originally had. A slight degree of adaptive modification may be observed 

 in the tarsus of Thoathermm, but as compared with the pes of the less 

 perfectly monodactyl horse, it is surprisingly incomplete. 



Aside from certain imperfectly known and doubtful forms, the Santa 

 Cruz Proterotheriidse are divisible into four well-marked genera, dis- 

 tinguished by the following characteristics. 



A. Feet tridactyl ; internal cusps of upper molars distinct ; lateral lower incisor (ij) much larger 

 than median (i^), which is very small. 



I. Nasals much shortened ; lower molars without pillar in posterior crescent ; last molar without 

 talon. Diadiaphorus. 



II. Nasals of normal length ; lower molars with pillar in posterior crescent ; last molar with 



talon. 



