SCOTT : LITOPTERNA OF THE SANTA CRUZ BEDS. I07 



theriidae ; the carpus has a similar semi-taxeopod arrangement, the peculiar 

 overlapping of the pyramidal upon the magnum seen in Macrauchenia 

 having not yet come about and the contact being still entirely lateral. 

 The metapodials are relatively longer and the phalanges shorter than in 

 the preceding family (except Thoatherium), and the unguals are short, 

 broad and heavy, not in the least equine in character. The feet are tri- 

 dactyl and nearly isodactyl, the three digits being of nearly equal length 

 and thickness. The tarsus is closely similar to that of the Proterothe- 

 riidse, but the astragalus has a slightly more perissodactyl appearance in 

 the less convex and more depressed head. 



In the Santa Cruz beds only two genera have been found which are 

 referable to the Macrauchenid^ and one of these, Adianthus, may prove, 

 when better known, to be more properly assignable to a different family. 

 This Ameghino has already proposed ('94^, 27), but until the genus is 

 more completely understood, it seems an unnecessary complication. 



1. Size of animal larger; lower molars with very conspicuous pillar and spur in 



posterior crescent. Theosodon. 



2. Animal very much smaller ; lower molars without pillar or spur in either 



crescent. Adianthus. 



THEOSODON Ameghino. 



(Plates XVI-XX.) 



Theosodon Ameghino; Enumeracion sistematica, etc., 1887, p. 19. 



"i PsetidocoRlosoma Ameghino; Rev. Argent, de Hist. Nat, T. I, 1891, p. 



294. 

 Oxyodontotherhtm Lydekker [non Amegh.); Anales del Mus. de La Plata, 

 T. II, 1893, p. 65. 



This genus which is the only well known Santa Cruz representative of 

 the family, is one of the commoner elements of the Santa Cruz fauna and is 

 quite fully represented in all the collections ; it is thus now possible to 

 make out almost every detail of dental and skeletal structure. Through- 

 out, there is a close resemblance to the much later Pampean genus, 

 Macrauchenia, though, on the other hand, there are many important 

 modifications, especially of the skull, in the latter. 



Dentition (Pis. XVII, figs, i, \a, 3, 3«, 4; XVIII, i, la; XX, i, \a). 

 — The dentition is complete and unreduced, having the formula, If, Ct, 

 Pf , Ml, = 44. 



A. Upper Jaw. The incisors are arranged in an almost straight, antero- 



