40 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 149 



men is high on the medial surface, a little below and a short distance 

 posterior to M 3 . 



The extensive breadth and depth of the angle of the lower jaw 

 indicates a large area of attachment for the masseter muscle externally 

 and the internal pterygoid muscle medially. These, of course, corre- 

 late with the depth and strength of the zygoma and the prominence 

 of the pterygoid fossa, and are important in the lateral movement of 

 the jaw for grinding as well as in raising it. The masseteric fossa, 

 however, is not deeply excavated, and its forward boundary is very 

 weakly defined. Dorsally a weak ridge extending upward and back- 

 ward from the anterior margin toward the sigmoid notch would seem 

 to distinguish between the areas of the masseter and temporal muscles. 



The anterior margin of the ascending ramus rises abruptly and high 

 to the dorsoventrally elongate and slightly backward curving coronoid 

 process. The coronoid process is not wide anteroposteriorly, and 

 the sharp sigmoid notch is followed immediately by the generally 

 convex and decidedly oblique condyle. The condyle has very little 

 neck and is situated well forward with respect to the posterior margin 

 of the angle. 



The lower jaws of other Eocene condylarths do not generally taper 

 forward so markedly through their length as in Meniscotherium, 

 although in some specimens of Hyopsodus the increase in depth pos- 

 teriorly was rather noticeable. The depth of jaw in Phenacodus and 

 Ectocion is relatively less and seems more nearly uniform beneath the 

 cheek teeth. In all these the jaws are strongly united at the symphy- 

 sis although the symphysis may not extend so far posteriorly with 

 respect to the cheek teeth as in Meniscotherium. Moreover, although 

 Phenacodus shows a rather prominently expanded angle, it seems 

 relatively less so than in Meniscotherium. Also, the masseteric fossa 

 is more noticeably excavated in all and better defined anteriorly than 

 in Meniscotherium. In a relatively well preserved mandible of 

 Hyopsodus it was observed in addition to the well-excavated mas- 

 seteric fossa that the coronoid process seems relatively broader antero- 

 posteriorly and not so high, also that the condyle has its long axis 

 distinctly transverse and is a little less convex across this diameter. 



The mental foramina in Phenacodus appear generally to be two in 

 number and placed about beneath P x and P 4 . This would appear to 

 be the case also for Ectocion in the limited material at hand. In 

 Hyopsodus, however, there are frequently, if not generally, three and 

 sometimes four foramina spread out between a position beneath P a 

 to P 4 or Ml 



