SCOTT : LITOPTERNA OF THE SANTA CRUZ BEDS. 83 



TICHODON Ameghino. 



(Plate VIII, Fig. 8.) 



Tichodon Amegh.; Enum. Synopt., etc., 1894, p. 43. 



TiCHODON QUADRiLOBUS Ameghino. 



(Plate VIII, Fig. 8.) 



Tichodon qtiadrilobus Amegh.; Enum. Synopt, etc., 1894, p. 43. 



Known only from a single tooth and of quite indeterminate position. 

 This tooth, ms^ of the left side, measures 14 mm. in antero-posterior, by 

 6.5 mm. in transverse diameter. 



Locality. — Not given. 



THOATHERIUM Ameghino. 

 (Plates XI, Fig. 12; XII, Figs. 3-i3«; XIII-XV.) 



Thoatherium Amegh.; Enumeracion sistematica, etc., 1887, p. 19. 

 Merycodon Mercerat; Rev. del Mus. de La Plata, T. I, 1891, p. 466 [fide 



Ameghino). 

 Rhagodon Mercerat ; Ibid., p. 468 [fide Ameghino). 

 Proterotherium Lydekker, in part ; Anales del Mus. de La Plata, T. II, 



1893, p. 63. 

 This genus represents the extreme modification among the Protero- 

 theriidae as regards digital reduction ; indeed, it is the most completely 

 monodactyl animal known, decidedly surpassing the horses in this respect. 

 However, the relatively isolated position of the genus is shown not only 

 in the feet, but in all parts of the skeleton also. It may be characterized 

 as follows: (i) The upper lateral incisors are caniniform, but smaller than 

 in any of the preceding genera. (2) In the upper molars the two internal 

 cusps and the anterior conule are connected by a continuous .crest, while 

 the posterior conule is vestigial, or absent. (3) The lower lateral incisor 

 is not caniniform and is hardly larger than the median. (4) The lower 

 molars have no internal pillar and mg is without a talon. (5) The nasals 

 are shorter than in Proterotherium, longer than in Diadiaphorus, and the 

 dorsal portion of the occiput is remarkably narrow and projects far behind 

 the plane of the condyles. (6) The limb-bones are all more slender and 

 elongate than in any of the preceding genera and the ulna and fibula are 

 more reduced. (7) The feet are strictly monodactyl, the lateral digits 



