FORT UNIOJ^ OF CRAZY MOUNTAIN FIELD, MONT. 149^ 



quadrangular, transverse trigonid basins, paraconids small and 

 closely approximated to metaconids, especially on M2-3; cusps. sub- 

 marginal; no metastylid. Talonid of M3 greatly enlarged, with strong- 

 third lobe with (at least) tv/o distinct, transversely paired cusps. 

 P^ 2-rooted. P* 3-rooted, with strong protocone, no distinct meta- 

 cone. Upper molars primitively tritubercular, \\dthout mesostyle, 

 protostyle, or hypocone, but with a ridge from the protocone swinging 

 around the posterointernal corner, which is much expanded and 

 basined. Internal bases generally bilobed. 



The morphology of the genus is described under its type species, 

 and the distinctive characters of the second species mentioned under it. 



PAROMOMYS MATURUS Gidley 



Figures 30, 31; Plate 7, Figures 2, 2a, 3, 3a; Plate 8, Figures 2, 2a, 3, 3a 

 Paromomys maturus Gidley, 1923, p. 3. 



Type. — U.S.N.M. no. 9473, right lower jaw with P4-M3 and anterior 

 alveoli. Collected by A. C. Silberling. 



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

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



Diagnosis. — Trigonids Mi -2 notably narrower than talonids. P* 

 strongly transverse. Internal bases of M^~^ strongly bilobed. Meas- 

 urements and derived statistical data given below. Dental formula 

 nM (but see below). All teeth closely placed, vidthout diastema. 



Discussion.— Theie are six specimens in which the anterior alveoli 

 are all shown, although in none are their rims unbroken. One of 

 these, as noted by Gidley (1923, p. 9 — Gidley mentions two, but the 

 other is not of this species), has a minute pit external to and between 

 the incisor and canine, which might be an alveolus for a vestigial 

 incisor. It could, however, be a mere break, and as none of the other 

 five specimens shows it this is a more probable explanation. I2 was 

 thus probably absent, and if ever present was vestigial and oftener 

 lacking. 



The incisor and canine are unknown except by their alveoli. The 

 incisor was large, its root slightly compressed laterally, and was semi- 

 procumbent, its root extending to beneath the posterior end of P2 

 or anterior end of P3. The canine was considerably smaller, its nearly 

 circular root with about half the (maximum) diameter of Ii. It is less 

 procumbent, and its root is shorter than that of Ii, the root of which 

 passes beneath it. The one canine root occupies about the same 

 space as the two of P2. 



P2 has two separated, divergent roots. The crown is high, slender, 

 very slightly procumbent, and somewhat recurved at the tip. The outer 

 face is convex, the inner excavated anteriorly and posteriorly, adjacent 

 to curving, vertical sharp anterointernal and posteroexternal crests.. 



