FORT UNION OF CRAZY MOUNTAIN FIELD, MONT. 



81 



KEY TO AMERICAN GENERA OF TERTIARY PTILODONTIDAE (LOWER JAWS) 



7. Lower incisor little or not compressed, relatively smaller, more 

 completely enameled: 



A. Anterior base of P4 notched: 



1. P4 longer than Mj, numerous serrations and strong ridges: 



a. Ml relatively longer, more cusps Ectypodus ^' 



b. Ml relatively shorter, fewer cusps Ptilodus ^' 



B. Anterior base of P4 not notched: 



1. P4 longer than Mi, numerous serrations and strong ridges. _ Parectypodus 

 II. Lower incisor compressed laterally, relatively larger, enamel 

 (below tip) more or less limited to a band: 



A. Base of P4 notched. 



1. P4 longer than Mi, numerous serrations and strong ridges-. Neoliotomus 



2. P4 shorter than Mi, few serrations and feeble ridges Microcosmodon 



B. Base of P4 not notched. 



1. P4, longer than Mi, numerous serrations and strong ridges. Eucosmodon 



KEY TO AMERICAN GENERA OF TERTIARY PTILODONTIDAE (PJ) 



7. P* with two complete cusp rows and rudiments of a third Ptilodus 



77. P^ with only one complete cusp row, with rudiments of a second: 



A. Main cusp row rising posteriorly in an elevated point, noticeably 



anterior to the posterior end of the tooth base Ectypodus 



B. Main cusp row approximately horizontal, or arched: 



1. Cusps of main row heavier, fewer, rudiment of second row 



stronger Eucosmodon 



S. Cusps of main row smaller, more numerous, rudiment of 



second slight Neoliotomus 



[P* not known Parectypodus, Microcosmodon 



The known distribution is shown in table 8. 



Table 8. — Known distribution of American genera of Plilodontidae 



Genus 



Ptilodus 



Ectypodus 



Parectypodus 



Microcosmodon. . 



Eucosmodon 



Neoliotomus 



Paleocenk 



Lower (Puerco, 

 lyower Fort 



Union of Clark 

 Fork Basin) 



Middle (Torre- 

 jon, Crazy Moun- 

 tain Fort Union 

 No. 2, Middle 

 Fcrt Union of 

 Clark Fork Basin) 



Upper 



A (Tiffany, Up- 

 per Fort Union, 

 Paskapoo) 



B (Clark Fork) 



Lower (Sand 



Coulee, Gray 



Bull) 



3' The distinction given may not be constant or really of generic value. The lower jaws of Ptilodus and 

 Ectypodus show no clear generic distinction, although P< shows them to be quite separate, although closely 

 related genera. 



