rORT UNION OF CRAZY MOUNTAIN FIELD, MONT. 157 



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

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



Diagnosis. — Sole known species of genus as redefined. See mor- 

 phological and numerical data (table 33). 



Discussion. — From their alveoli, in two specimens (9532 and 9534), 

 Ii and C are much as in Paromomys maturus, but the incisor is perhaps 

 relatively a little larger and the canine still smaller, its alveolus 

 occupying distinctly less space than the alveoli of Po. There is no 

 evidence of a second incisor. 



The alveoli of P2 are confluent, and in one specimen the roots are 

 fused, in another barely separate. P2 and P3 are similar to each other 

 and to those of P. maturus. P2 is slenderer and slightly higher than P3 

 and P3 has the incipient anterior cuspule more nearly distinct than Po. 



P4 is much more progressive than in Paromomys, having small but 

 distinct subequal paraconid and metaconid in all cases, whereas in 

 Paromomys these cusps are either barely incipient or entirely absent 

 on P4. The talonid and general structure are, however, as in Paro- 

 momys. 



Ml almost exactly resembles that of P. maturus. M2-3 are also 

 closely similar but have the paraconid more distinct than is usual in 

 Paromomys, lower on the crown and also a little more external, or less 

 marginal. On both Mi and M2 the trigonid is nearly as wide as the 

 talonid. On Mi_3 the trigonid is more elevated than in Paromomys, 

 and the external cingulum is weak or absent on the talonid. The third 

 lobe of M3 is much less developed than in Paromomys maturus, but 

 the hypoconulid is bifid or, in one case (9430) approximately trifid. 



The mandible is also much as in Paromomys. In one specimen 

 (9450) most of the posterior part is present, although the ends of the 

 three processes are broken. The inner face is nearly plane. The 

 coronoid is broad and apparently rose little above the articular process. 

 Its anterior border is nearly straight and at right angle to the alveolar 

 border. The condyle is far above the molar level. The large dental 

 foramen is far posterior to the teeth, beneath the corono-condylar 

 notch, and is above the alveolar level. The angle is long, slender, 

 styliform, and thickened, and extends backward and slightly down- 

 ward. Its tip was posterior to a vertical from the condyle. 



P* is not known, but it has three roots and was nearly as wide as M^ 

 The upper molars closely resemble those of Paromomys maturus 

 except in being somewhat more transverse, with slenderer sharply 

 pointed cusps, and in the much less marked posterointernal expansion 

 and basining. There is an internal vertical groove, but on M' the 

 base is not bilobed and on M^ this is barely indicated. Several of 

 these points are resemblances to Paromomys depressidens, and it has 

 already been noted that the teeth on which that species is based are 

 much worn and of doubtful detail. As the two species are of about 



