NO. 6 PALEOCENE FAUNAS OF BISON BASIN — GAZIN I3 



that the form represented is Ptilodus, as no other portions of the 

 dentition were found and the anterior margin of the tooth was broken 

 away. In size and outhne, as well as in the spacing of the serrations, 

 of which there were at least 13, the tooth rather closely resembles P4 

 in Ptilodus montanus. The preserved portion measures 8.1 mm., but 

 estimated from complete specimens of P. montanus this tooth in its 

 entirety would have been about 8.8 or 8.9 mm. long and within the 

 upper limit of measurements for P. jnontanus. 



Cf. ECTYPODUS MUSCULUS Matthew and Granger, 1921 



Plate I, figure 3 



A small jaw fragment with Mi and the alveoli for M2, U. of Wyo. 

 No. 1105, would appear to represent Ectypodus miisculus. The size 

 of the included molar, 2.5 by 1.25 mm., is near that given by Granger 

 and Simpson (1929, p. 655) for E. iiiuscnlus, although the Wyoming 

 specimen would appear to be about a quarter of a millimeter broader. 

 The cusp formula, 9 : 5 or 6, is comparable to the 8 : 6 cited by Jepsen 

 (1940, p. 307) as well as by Matthew and Granger, particularly as 

 one of the outer cusps in No. 1105 is scarcely distinct. This tooth 

 may well belong to the form represented by the P4, U.S.N.M. 

 No. 20878, compared below with Ectypodus hascni but is appreciably 

 shorter than the 3.2 mm. cited by Jepsen for the length of Mi in 

 E. haseni, although the cusp formula for this tooth is the same as that 

 for E. mus cuius. 



Cf. ECTYPODUS HAZENI Jepsen, 1940 

 Plate I, figure 2 



An isolated though complete P4, U.S.N.M. No. 20878, compares 

 very closely to this tooth in the Silver Coulee Ectypodus haceni. It 

 resembles this form in the size (5 mm. long) and outline of the tooth 

 but has only 11, or possibly 12, serrations rather than the 13 listed 

 by Jepsen (1940, p. 307). No. 20878 also resembles P4 in Mimetodon 

 churchilli, which is indicated as having 12 serrations, but the tooth 

 has perhaps a somewhat more convex profile, with the straight pos- 

 terior section a relatively shorter part of the entire profile. The an- 

 terior margin of the tooth is deeply notched and pocketed, suggesting 

 the presence of P3, but the absence of other associated material pre- 

 cludes certain generic identification. 



