74 Bulletin American Museum of Natural History. — [Vol. XXXIV, 
Gray Bull Beds, Big Horm Basin. Nos. 15742, 15515, 15734, 15743, lower jaws, and 
three unnumbered jaws from the Shoshone River in the Big Horn Basin are referred to 
this species. 
Distinctive characters: Slightly smaller than hians and opisthotoma. Heels of all 
lower molars large. Surface of enamel rugose striated vertically, canines heavily 
Wo, 1/6/75 
Zeid 
Fig. 65. Sinopa shoshoniensis, lower jaw, type specimen, natural size, outer view and 
crown view of teeth. Gray Bull beds, Clark Fork Basin, Wyoming. 
grooved. Premolars robust without anterior basal cusps. Heel of last lower molar 
broader and shear of trigonid more transverse than in opisthotoma. M2 nearly as 
large as m!, p? with strong inner cusp. Posteroexternal cusps of pms small, antero- 
external rudimentary. 
This species is nearly related to opisthotoma, but distinguished by the 
typical proportions of the molar shears; it is evidently allied to the typical 
group of Sinopa (S. rapax, pungens and grangert) of the Middle Eocene, and 
may well be ancestral to it. 
Sinopa strenua (Cope 1875). 
Prototomus strenuus Corn, 1875, Syst. Cat. Eoc. Vert. New Mex., p. 10; (Stypo- 
lophus), 1877, Ext. Vert. New Mex., p. 117, pl. xxxix, fig. 11; (Stnopa) Matruew 1901, 
Bull. A. M. N. H., Vol. XIV, p. 26. 
Sinopa hians (Cope), Marrurw 19011. c. Not Stypolophus hians of Cope. 
Type, U. 8. Nat. Mus. No. 1023, lower jaws with Ps-m; r. and |., much damaged 
- and buried in matrix. ? 
Distinctive characters: m'!— = 22 mm.; mi_3 = 24-26 mm. ; heels of molars small, 
trigonids high; jaw long and slender anteriorly, pi two-rooted, pz not spaced. Enamel 
smooth. 
A number of specimens from the Gray Bull horizon of the Big Horn 
Wasatch agree fairly well with the type of S. strenua. No. 15234, anterior 
half of skull and lower jaws, and No. 2850 upper and lower jaws with frag- 
