208 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxx. 



paracones and metacones of upper molars well separated, heels of 

 lower molars much larger than in T. ag/'/is, metacone on M- well 

 developed. Skeleton smaller with shorter limb-bones. 



Teeth, (ligf. l/>, 2(?) Dentition f^y^- Incisors small, transverse, 

 canines slender and of moderate size. Premolars trenchant, moder- 

 ateh" compressed, F- two-rooted, but of subtriangular outline with 

 rudimentary internal cusps. P^ three-rooted with large lunate internal 

 cusp, small antero-external and larger postero-external basal cusps 

 and massive conical protocone. Molars functionall}' resembling those 

 of the opossum, but only three in number, with principal oblique and 

 subordinate transverse shears, the upper ones of triangular outline, 

 paracone and metacone of equal size and well separated, large lunate 

 antero-internal protocone, and broad external cingular shelf. M^^-^ 

 sub-ecjual, with small paraconule and metaconule, parastyle small, meta- 

 style extended into a strong shearing blade. M^ smaller, transverse, 

 metacone well developed but smaller than paracone, no metast3de, para- 

 style extended into a short shearing l)lade, no metaconules. Lower 

 molars of nearly equal size, but My smaller than the others, the trigo- 

 nids high, of triangular form, paraconid and metaconid well developed, 

 sul)-cqual, protoconid overtopping" both. Heels basin-shaped, as large 

 as the trigonids, except on M^. 



In 8. rapax (tig. 1«, 2c/) the internal cusps of P^ to ]M^ are more 

 compressed and less broadly lunate, the principal cusp of P^ is less 

 massive, the heels of the lower molars are smaller, and there is no 

 diastema behind P^. In T. agiJis (fig. le, 2/") the internal cusps of 

 the upper teeth are smaller and much more compressed, P- is com- 

 pressed and trenchant, the paracone and metacone are closel}" connate 

 and of unequal size, metacone absent on M-, heels of lower molars 

 much smaller, metaconids greatly reduced, etc. In T. irhithr (tig. 

 1/', 2(/) the inner cusps of the molars are extended inward, the meta- 

 conids well developed, but otherwise it is much as in T. agiUs. 

 The Lower Eocene species exhibit a further approach toward the true 

 Shiopa in one or another feature, but all are nearer to S. rupcu- than 

 to the species here described. S. major of the Bridger (fig. Ic, 2(^) 

 is larger and more massive than S. granger^ with lower crowned teeth, 

 broad heels to the lower molars, etc. 



SKULL, DESCRIPTION AND COMPARISONS. 



The skull is elongate in both facial and cranial regions, with narrow 

 nuiz/le, small brain case, moderately high sagittal and occipital crests, 

 short and rather slender arches, strong postorbital constriction. The 

 premaxilla' are deeply excavated for the reception of the lower canines, 

 the borders of the excavation very strongly marked; their ascending 

 processes are ver}^ slender and extend backward only to a point above 

 PJ-. The nasals are slightl}' expanded in front, and somewhat more 



