151 BULLETIN 16 9, UNITED STATES NATIONAL MUSEUM 



The infraorbital foramen is high and narrow and lies immediately 

 anterior to the anterior root of P*. Very little of the orbital rim is 

 preserved, and I see no basis for supposing it larger or smaller than in 

 any possibly related group. It seems probable that it extended little, 

 if any, farther forward than P*. 



The principal numerical data on lower teeth of this species are given 

 in table 30 (see also fig. 3). 



The highly homogeneous character of the sample, and by inference 

 the only slightly variable nature of the species, is very striking. All 

 the coefficients of variation are remarkably low and even the highest, 

 5.16 ±0.86, is very commonly exceeded in races that are pure in the 

 strictest sense. 



There are too few upper jaws to calculate derived statistical data. 



PAJROMOMYS DEPRESSIDENS Gidley 



Figure 32; Plate 9, Figure 7 

 Paromomys depressidens Gidley, 1923, p. 4." 



Type. — U.S.N.M. no. 9546, part of right upper jaw with P^-A'P. 

 Collected by A. C. Silberling. 



Horizon and locality. — All known specimens from Gidley Quarry, 

 Fort Union, Middle Paleocene horizon. Crazy Mountain Field, Mont. 



Figure Z2.— Paromomys depressidens Gidley, U.S.N.M. no. 9485, left lower jaw, crown view. Four times 

 natural size. (After Gidley, 1923, fig. 3.) 



Diagnosis. — Trigonids of Mi_2 nearly as wide as talonids. P^ less 

 transverse than in P. maturus. Bases of M^~^ less strongly bilobed. 

 Size notably smaller; length M2 (mean of three specimens), negative 

 deviation from mean in P. maturus about eight times standard devia- 

 tion of latter. See also measurements in table 33. P4 larger relative 

 to Mi; ratio LP4:LMi (one specimen), positive deviation from mean 

 in P. maturus over three times standard deviation of latter. M2 wider 

 relative to its length; ratio LM2:WM2 (mean of three specimens), 

 negative deviation from mean in P. maturus nearly three times stand- 

 ard deviation of latter. 



Discussion. — Ii-Ps are loiown only from their alveoli, which are 

 developed about as in Paromomys maturus. Of the two specimens 

 showing these alveoli, one (9416) has a possible alveolus for a vestigial 

 I2, and the other (9482) does not, so that, as in P. maturus, this tooth 

 was either absent or inconstant. P4 is slenderer and somewhat less 



6< In referring to Gidley's description, note that the text has been transposed. The text in Gidley, 1923, 

 from p. 5, line 12, beginning "Several upper -jaw portions . . ." to p. 6, line 3, ending ". . . above the junc- 

 tion of P3 and P'" is made part of the description of P. depressidens but in fact refers to P. maturus and evi- 

 dently was meant to follow p. 4, line 12, of text, after ". . . as in Notharctus nunienus (Cope)." 



