SCOTT : LITOPTERNA OF THE SANTA CRUZ BEDS. 6 1 



a more rounded, less angular outline : the facet for the astragalus is nar- 

 rower and somewhat less concave and is bounded externally by a larger 

 and deeper sulcus, while the cuboid facet is somewhat less oblique and 

 concave. The articular surface for the fibula is small and irregular in 

 shape. The navicular resembles that of Diadiaphorus, except that the 

 proximal surface contracts less on the plantar side. The entocuneiform is 

 very large , but compressed and scale-like, attached to the navicular later- 

 ally rather than proximally and articulating extensively with the second 

 metatarsal. The mesocuneiform is very small in all three dimensions, 

 being much more reduced, especially in proximo-distal length, than in 

 Diadiaphonis. The large and heavy ectocuneiform has nearly the same 

 proportions as in the latter, except that the facet for mt. Ill narrows less 

 abruptly toward the plantar side. 



The metatarsus shows some noteworthy differences, though of a very 

 minor character, from that of Diadiaphorus. Owing to the reduction of 

 the mesocuneiform, the second metatarsal rises higher above the head of 

 mt. Ill than in the latter and has a much more extensive lateral contact 

 with the ectocuneiform and also extends farther distally, so that, as a 

 whole, it is considerably longer than in the genus last named. Metatar- 

 sal III is somewhat longer and considerably stouter than metacarpal III, 

 thus reversing the relative lengths which occasionally are found in Diadia- 

 phonis and the difference in breadth is also greater than in that genus. 

 I have seen no complete specimens of mt. IV, the proximal end being 

 absent; distally it extends to the same level as mt. II. 



The proximal phalanx of the median digit is of the same length as that 

 in the manus, or, it may be a little shorter, somewhat thicker and less 

 depressed planto-dorsally. The second phalanx is likewise thicker and a 

 little longer than in the manus. Of the ungual I have seen no example. 



Species. — Of all the Santa Cruz genera of this family, Proterotherium 

 is the most varied and abundant, and was evidently in a state of vigorous 

 development. This renders the discrimination of species exceedingly 

 difficult, and for no other genus of the Litopterna does the absence of any 

 minutely exact stratigraphical knowledge of the Santa Cruz beds have a 

 more unfortunate effect in confusing the taxonomy. Most of the species 

 are very variable and it is rare to find two individuals that seem to be 

 specifically identical. There is no ground for surprise, therefore, to find 

 that among the casts of the Ameghino collection there is frequently as 



