NO. 8 UPPER EOCENE ARTIODACTYLA — GAZIN 23 



we involve ourselves in a very doubtful and changeable arrangement, 

 dependent upon hypotheses of relationship instead of facts of struc- 

 tural affinity." As explained later, the name Helohyinae is used rather 

 than Zittel's name Achaenodontinae. 



Subfamily Homacodontinae Peterson, 1919 



Two of the dichobunids listed by Simpson (1945) as uncertain in 

 position, Microsus and Antiacodon, would both appear to be homa- 

 codonts. Microsus, the oldest of the North American dichobunid 

 names, is clearly to be placed among the homacodonts. Comparison 

 of the type of Microsus cuspidatus Leidy with the lower jaw belonging 

 to the type of Homacodon vagans Marsh reveals little to distinguish 

 these forms but size. The Microsus cuspidatus type has teeth ap- 

 proximately 20 percent smaller and, in addition, the cusps appear to 

 be relatively high and more acute, with the hypoconid and entoconid 

 better separated. However, the teeth in the M. cuspidatus type are 

 less worn than in H. vagans, and referred material of M. cuspidatus 

 has the hypoconid and entoconid less well separated. Moreover, 

 Microsus shows greater tendency to retain the paraconid. It is usually 

 distinct on M^, variable on M2, and usually absent from M3. Homaco- 

 don is in all probability a genus distinct from Microsus but there is 

 rather little to show this in posterior lower molars. In referred ma- 

 terial of M. cuspidatus it should be noted that P4 exhibits a metaconid 

 not seen in H. vagans, suggesting that the later homacodonts may be 

 more closely allied to Microsus than to Homacodon. Certain unde- 

 scribed upper molars in the Bridger collections of the American Mu- 

 seum (Nos. 12146 and 12696), cataloged as questionably Homacodon, 

 are almost certainly of Microsus. They agree structurally in almost 

 all details with Homacodon vagans but are distinctly smaller and the 

 cusps are more delicate and acute, as they would be in Microsus. 



The genus Antiacodon was based on the species A. venustus Marsh, 

 but careful comparison of type materials shows this species to be a 

 synonym of Sarcolemur pygmaeus (Cope) {=S. furcatus Cope), 

 likewise a genotype. Nevertheless, the generic name Antiacodon has 

 priority so that the type species becomes Antiacodon pygmaeus. Re- 

 cently (Gazin, 1952, p. 73) the record of the Antiacodon stem has 

 been extended down into the lower Eocene by the discovery there of 

 Hexacodus. The trigonid of the lower molars in Hexacodus shows 

 Antiacodon peculiarities in an incipient stage but with less difference 

 from the structure of this part seen in Microsus. The talonid of the 

 lower molars, with particular attention to the hypoconulid, is very 



