SCOTT : LITOPTERNA OF THE SANTA CRUZ BEDS. II3 



few and small in amount. In M. patachonica, the upper incisors are no 

 longer arranged in an antero-posterior line, but form a transverse series, 

 and have become relatively smaller and more chisel-shaped. In both 

 jaws, the incisors have an enamel pit, or "mark," somewhat resembling 

 that of the horses, and formed by the great growth of the internal cingu- 

 lum, which is in a merely incipient stage in Theosodon. Bravard's figures 

 (Burmeister, '64, PI. I, figs. 2, 3) appear to show that the upper canine 

 was two-rooted. The premolars are little or not at all more complex than 

 in Theosodon. The upper molars are not quite so strictly brachyodont 

 as in the Santa Cruz genus ; the external crescents are more angular and 

 more deeply concave, and the grinding surface is somewhat more com- 

 plicated, with three or more enamel lakes in the moderately worn stage, 

 but the change from Theosodon is not great. The lower molars are sim- 

 plified by the suppression of the pillar in the posterior crescent, but 

 Ameghino states that it is present in the milk-teeth ('94<5, 279). 



Skull (PL XVII, figs. I, la, id). — The skull of Theosodon is extremely 

 interesting and peculiar and differs much more from that of Macranchenia 

 than would be expected from a comparison of the teeth alone. As in all 

 the other known genera of this family, the skull of Theosodon is long, narrow 

 and low, but the dorso-ventral shallowness is, in most specimens, exag- 

 gerated by the vertical crushing and distortion to which the skull has been 

 subjected. It is rare to find an individual which has suffered neither 

 lateral nor vertical distortion. Both cranium and face are elongate, the 

 anterior margin of the orbit, which is above the middle of m-, forming 

 nearly the median point in the total length of the skull. The upper profile 

 of the skull rises but little from the occipital crest to the parietal eminence 

 and thence descends very slightly and gradually to the end of the nasals, 

 from which point it slopes down very rapidly to the low rostrum. In 

 Macranchenia, the parietal eminence is higher and farther forward and 

 from it the upper profile descends in an almost uninterrupted slope to the 

 end of the rostrum, the minute, vestigial nasals forming the only break in 

 the line. 



The occiput is quite high and narrow, broadest at the base and rapidly 

 contracting dorsally ; above the foramen magnum, the surface is strongly 

 convex transversely; the foramen itself is quite large and of nearly circular 

 shape. The occipital crest is low and thin and passes uninterruptedly into 

 the dorsal border of the zygomatic process of the squamosal, thus com- 

 pletely defining the temporal fossa behind. 



