92 



GREGORY: NOTHARCTUS, AN AMERICAN EOCENE PRIMATE 



lar, the neck more constricted, the external condyle or trochlear keel is widely expanded. These char- 

 acters are probably associated with the freer movement at the acetabulum and superior leaping power 

 of Propithecus. 



The femur of Cebus differs radically from that of Notharctus in many characters: the third trochanter 

 , is obsolete or entirely absent, the head is spherical, the neck sharply constricted, the great trochanter 

 is more in line with the shaft and the width of the whole proximal part of the bone is relatively less; the 

 second trochanter has a much thicker tip, the digital fossa is much deeper, the adductor groove on the 

 back of the femur is barely indicated, the whole distal end is widely expanded, the patella face is wider 

 and the trochlear keels lower. These characters are accentuated in Alouatta, Avhich has also a widened 

 shaft. 



These differences in the femora of Notharctus and of Cchus are doubtless correlated with differences 

 in the musculature and in the action of the limbs. In Notharctus the stout gluteus maximus was doubtless 

 attached as it is in lemurs, not only to the third trochanter, but also all down the shaft below the third 

 trochanter; and the quadratus femoris, which is inserted on the back of this process is a stout muscle. 

 In Cehus the gluteus maximus muscle is less developed, especially the lower part, and the quadratus femoris 

 is weaker. In Lemur and Notharctus the narrow patellar facet and high trochlear keels tend to limit 

 the motion of the leg at the knee to an anteroposterior plane. In Cebus, on the contrary, the wide patellar 

 facet and low trochlear keels permit a greater twisting of the leg upon the femur. All this is correlated 

 with the fact that Lemur and Notharctus were specialized for leaping, while Cebus has become somewhat 

 de-differentiated and has more mobile limbs. These differences are still more accentuated in Alouatta, 

 which has the great trochanter small, the third trochanter absent, the distal end very wide and permit- 

 ting wide twisting at the knees. In the marmosets the femur is essentially of cebid type (Fig. 15.5). 



Patella 

 Text Imu-. 17 



The patella is represented in N. osborni (No. 11474) and in tyrannus (No. 11478). It is a small 

 nodule of l)one narrower than the patella of Lemur and still narrower than those of Cebus and Alouatta, 

 in confoi'mity with the narrowness ot the patellar facet in the femur. It is much less produced below 

 than that of Lemur. Its lower end is asymmetrical while that of Cebus is more symmetrical and rounded, 

 in accordance^ with the greater freedom of the knee-joint. 



Ill Propilhecus the patella is essentiall.\' similar to that of Lemur. 



Tibia 



Text Fig-, 17 



Both tibiae are well preserved in N. osborni. No. 11474; the right tibia is nearly complete in N. tene- 

 brosus (A. M. 1727); several other less complete tibiae are known. As compared with those of Lemur 

 mongoz, they are much shorter, more robust, and more curved, especially in the anteroposterior plane. 

 There is a prominent eminence, or tuberosity, on the front face of the tibia, about one-third of the way 

 down the shaft, which may be designated as the "lower anterior tuberosity" of the tibia (the cnemial 

 tuberosity being the "anterior tuberosity" of human anatomy): the lower anterior tuberosity is much 

 better developed in Notharctus than it is in Lemur. In Lemur this tuberosity is farther dorsad; it lies 

 beneath the upper part of the strong fascia which covers the tibialis anticus muscle, and is especially 

 associated with the Inmd-like tendf)n of the conjoined sartorius and gracilis muscle, which passes over 



