52 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I49 



major ridge as distinct and extending on the proximal two-fifths of 

 the length. This I find, however, is variable and usually weak or 

 absent but noted it observably developed on two specimens of M. 

 robustum. There is a generally shallow but well-defined fossa or 

 depression, which may be pitted or nearly flat, on the posterior 

 surface or margin of the greater tuberosity at the proximal extremity 

 of the deltoid ridge, evidently the fossa for the teres minor that Cope 

 mentioned as being flat. 



The distal extremity of the humerus is broad with a very well 

 developed inner condyle exhibiting an entepicondylar foramen which 

 is relatively larger in the smaller species M. chamense. The inner 

 condyle shows a relatively rough or rugose medial and distal margin 

 for the various flexor and certain other muscles to the manus. The 

 external condyle, however, is not nearly so projecting but is roughly 

 pitted for the extensors. The trochlea has an outstanding medial 

 margin anteriorly and distally and a posteriorly raised lateral margin 

 which arises from the nearly median convexity of the trochlea as it 

 extends around toward the posterolateral side, as noted by Cope. 

 With a posterior root continuous with this posterior lateral crest of the 

 trochlea and with an anterior root originating on the external con- 

 dyle, a flaring supinator ridge extends prominently upward and some- 

 what backward for about the distal third of the shaft. Above the 

 trochlea, as noted in several specimens, the bone seems incomplete, 

 leaving usually a broadly open supratrochlear foramen. 



The humerus of Phenacodus was illustrated by Cope (1884b, pi. 

 5"g> fig- 2) and briefly described. It is surprisingly like that of 

 Meniscotherium in a number of details, although it differs noticeably 

 in its generally straighter and relatively somewhat stronger appear- 

 ance. The proximal tuberosities seem a little more robust and anteri- 

 orly more projecting. The bicipital groove is relatively broader and, 

 as Cope noticed, there is a low ridge medially placed in the bicipital 

 groove, which is not seen in Meniscotherium. The deltoid and 

 supinator ridges have about the same relative extent as in Menisco- 

 therium, but the former crest seems somewhat less sinuous. The 

 distal extremity appears to be less expanded transversely, but the 

 trochlea has relatively greater anteroposterior diameter than in Menis- 

 cotherium. The details of the marginal crests of the trochlea, how- 

 ever, are somewhat alike in the two forms. 



In Hyopsodus paulus the proximal extremity of the humerus exhib- 

 its much less developed tuberosities. The greater tuberosity is more 

 rounded, less crestlike, and does not extend proximally above the 



