NO. 8 UPPER EOCENE ARTIODACTYLA — GAZIN 53 



pumilus primarily by its significantly smaller size. In addition to 

 this there are slight differences in the structure of the teeth that may 

 be regarded as representing a stage somewhat more primitive than is 

 evident in the P. pumilus dentitions. In P* the primary cusp is 

 twinned but less markedly so than in much of the P. pumilus material, 

 and the talon portions of both P^ and P* are slightly more constricted 

 anteroposteriorly. It is also noted that the cheek teeth, both upper 

 and lower, are perceptibly less selenodont. The cusps are just a little 

 more bunodont, and the outer wall of the upper molars has slightly 

 better-developed ribs on the paracone and metacone. Protoreodon 

 parvus would appear to be ancestral in every way to Protoreodon 

 pumilus and is somewhat closer to the type material of P. pumilus 

 than it is to the referred material of My ton pocket, indicating a 

 gradational sequence in which an arbitrary separation might be made 

 corresponding about to the transition between Uinta B and Uinta C 

 time. It should be noted, however, that a few of the larger protoreo- 

 dont specimens from White River pocket (Uinta B) seem difficult 

 to distinguish from P. pumilus and might also be confused with 

 Diplobunops vanhouteni. 



The type of Hyomeryx breviceps Marsh is almost indistinguishable 

 in the upper cheek teeth from Protoreodon parvus. The only possibly 

 significant character brought forth in defense of Hyomeryx breviceps 

 was absence of upper incisors. I was unable to find this portion of the 

 specimen in the collections at Yale, but there appears to be some 

 doubt as to Marsh's interpretation, inasmuch as Thorpe noted that 

 this portion of the premaxilla was broken down and that there may 

 have been a small incisor. In all the Protoreodon material that I have 

 examined where the alveolar portion of the premaxilla was well pre- 

 served, the three incisors were present, increasing in size from a very 

 small P to a moderate P. Only in certain damaged specimens was 

 there any doubt as to the presence of all. However, inasmuch as the 

 anterior incisors are relatively small, it would not be surprising if 

 they were occasionally missing, possibly in some instances through 

 damage and loss during the life of the individual. This portion is 

 apparently not preserved in the type of P. parvus. Among other 

 characters attributed to Hyomeryx breviceps, the absence or weakness 

 of the internal cingulum is probably of doubtful significance, but in 

 any case does not differ in this respect from the type of P. parvus. 

 The character of the styles noted by Thorpe for the upper molars of 

 H. breviceps is in part surmise, as all but the parastyle on M^ are 

 broken off. Marsh's illustration (1894, fig. 19) of these teeth gives 

 the wrong impression, inasmuch as the damage is not indicated and 



