88 BULLETIN 16 9, UNITED STATES NATIONAL MUSEUM 



complex. The external cusps are rounded on the external face and 

 have the more flattened internal face marked by a few deep radial 

 furrows and intervening ridges. The internal cusps are simpler but 

 tend to develop the same form, the furrowed side being external 

 (toward the middle of the tooth in both cases). The cusps of the 

 middle row are vaguely crescentic, the anterior face somewhat con- 

 cave and the posterior convex, with the two sides flattened and 

 furrowed. 



M^ is much shorter and very slightly wider than M'. Internal and 

 median rows are of about equal length, but the cusps of the middle row 

 are larger, fewer, higher, and more separate. They are more distinctly 

 crescentic than on M\ The outer row is confined to the anterior half 

 of the tooth and generally has a single crest and outer surface, so that 

 separate cusps cannot be distinguished. 



The sole lower incisor is a long, slender, curved, scimitarlike tooth 

 with a completely enameled crown, the enamel thin on the postero- 

 basal part and there not extending so far down. The anteroexternal 

 face is smooth and convex, and there is a sharp anteromedial (or 

 buccodistal) crest, next to which the internal face is excavated. There 

 is a much weaker and shorter but similar posteroexternal (bucco- 

 proximal) crest. The long, but closed, root is inserted in a heavy 

 collar of bone. 



Ii is followed by a long diastema, and homologues of Pi. 2 of the 

 Plagiaulacidae are absent. P3 is a tiny, 1 -rooted, styliforra tooth, 

 nearly circular in horizontal section, inserted vertically under the an- 

 terior edge of P4 in such a way that its crown fits tightly into a notch 

 in the base of the latter. The crown is slightly expanded and bulbous 

 and is enameled on the anterior face. The tooth has no function save 

 that of buttressing P4. 



P4 is the famihar large shearing tooth, which reaches its greatest 

 known development in this genus. It has been so often described and 

 so well figured as to require no detailed description here. Mi is a long, 

 narrow tooth with two cusp rows. The cusps resemble those of the 

 external and internal rows of M^ but tend to be very vaguely crescen- 

 tic, concave on the posterior surfaces. M2 is wider but much shorter. 

 The cusps are larger but less separated, in each row, and the median 

 valley is wider and more open. The external cusp row generally ex- 

 tends farther posteriorly than the internal. This tooth seems to suffer 

 more severe wear than does Mi. 



Skull. — Seen from above, the skull is almost perfectly triangular 

 except for the slight concavity in outline anterior to the zygoma. The 

 orbits are almost exactly median. The skull proper is broadest, and 

 about equally broad between the anterior and between the posterior 

 zygomatic roots, that is, between the anterior edges of the orbits and 



