l8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 128 



ing to compliance with cited characters. One should bear in mind that 

 even the best of keys are essentially arbitrary and often misleading, 

 and it is with some trepidation that the following is included in this 

 study. 



A. Cheek teeth essentially bunodont DICHOBUNIDAE 



1. Hypoconulid of Mi and Ma developed from cingulum distinctly posterior 



to saddle between hypoconid and entoconid. Pi, C, and probably I3 

 subequal as far as known Homacodontinae 



a. Trigonid somewhat elevated and anterointernal cusp of lower molars 



twinned (or paraconid distinct from metaconid). P4 with metaconid. 

 Talonid of lower molars broadly basined. 



(1) Paraconid and metaconid about equal and close together 



Hexacodus 



(2) Metaconid more reduced than paraconid and crista obliqua joins 



higher on metaconid. Upper molars with hypocone (may be 

 double), small cusp anterior to protocone, and small mesostyle 

 on external cingulum Antiacodon 



b. No paraconid on lower molars, except possibly M3, P4 without meta- 



conid. Cusps noncrescentic. M^ and M* with parastyle but no meso- 

 style. 



(i) Hypocone on M* and M* Homacodon 



(2) No hypocone on upper molars (lower molars not known, see also 

 id (2) (b) ) Mesomeryx 



c. Paraconid on Mi and vestigial or absent on other lower molars. P4 



with metaconid. Cusps high and noncrescentic Microsus 



d. Lower molars with subcrescentic outer cusps. P4 with metaconid. 



(1) Upper molars with mesostyle and distinct protoconule. 



(a) Hypocone on M' and M^ Marked diastemata separating ante- 



rior premolars Mytonomeryx 



(b) Hypocone on M* only (vestige on M''). Short or no diastemata 



separating anterior premolars Bunomeryx 



(c) No hypocone on upper molars. Short or no diastemata between 



anterior premolars Pentacemylus 



(2) Upper molars without mesostyle (or very weak). Protoconule 



indistinct or forming a loph with protocone. 



(a) Hypocone on M' and M^ Hylomeryx 



(b) No hypocone on upper molars. (Lower molars not known, see 



also lb (2) ) Mesomeryx 



2. Hypoconulid of Mi and M2 weak (and double) in early stages and absent 



in later stages. C not known in early stages but caniniform in ad- 

 vanced stages. P4 without metaconid. Hypocone of upper molars vesti- 

 gial or absent Heloh yinae 



a. Four lower premolars? Paraconid distinct from metaconid. Hypo- 

 conulid of Ml and Mj weak and may be developed from cingulum 

 posterior to saddle between hypoconid and entoconid, and as a sec- 

 ond small cuspule in the saddle very close to the hypoconid. 



( 1 ) Lower premolars in close sequence Helohyus 



(2) Lower premolars separated by marked diastemata Lophiohyns 



