98 



GREGORY: NOTIIARCTUB, AN AMERICAN EOCENE PRIMATE 



Calcaneum 

 Plate XXXI; Text Fig. 21 



While the joint between tlie tibia and fibula and the astragalus serves as a hinge, the calcaneum 

 serves as a lever to throw the body forward in leaping. We may, therefore, consider the tibio-astragaloid 

 joint as the fulcrum of a lever of the first class, the pull of the muscles of the calf as the power, and the 

 reaction of the supporting medium against the thrust of the foot as the resistance. In order to gain great 

 speed the fulcrum would be placed near the power and so we find that the power-arm of the lever, repre- 

 sented by the length of the tuber calcis, is short, while the resistance arm, which is the distance from the 

 astragalo-tibial joint to the ground, is long. This relation, already established in Notharctus, is empha- 

 sized in many modern lemurs and carried to an extreme in Tarsius and the Galagos. 



The calcaneum of Notharctus is fully as lemur-like as is the astragalus. The back of the tuber calcis 

 is ovoid, with a pointed upper end, and a dorsoventrally convex surface for the tendo Achillis. The 



4 5 6 



1 2 3 



Fig. 21. ( "oinijanitivf scries: left astragalus of y<}tli(ircfu.s jrin/iKi.v, " Addjiis iKirisini.si.s,'" and Vcliicoduft friKjicornff. X §. 



1. Notharctus ■pwj II ax, XnwY. Mus. No. 11721. Outer .side. 



2. "Adapis parisiensis." Amer. Mus. No. 10016. Outer side. 



3. Pdycodus frugivorus. Amer. Mus. No. 16852. Outer side. 



4. 5, 0. The same specimens, front, or dorsal view. 



inner side of the tuber is concave where it curves around the tendon of the flexor longus hallucis. The 

 ectal and sustentacular facets are substantially similar, and so is the interosseous groove between them. 

 The wdde lower or anterior part of the upper surface is roughened for the external calcaneo-astragaloid 

 ligament and for the short extensors of the digits. The lower or distal end has a concave facet for the 

 cuboid and in some specimens there is a pit for the calcaneo-cuboid Ugament. The outer side of the 



