(jregory: notuaiu'tus, an American eocene primate 



97 



This conformation of the head permits of a certain amount of twisting of the navicular upon the astraga- 

 lus. The articular surface of the head is somewhat produced upward on to the front face, but this part 

 rubs on the under side of a ligament running from a certain tubercle on the front of the neck of the astrag- 

 alus to the dorsal surface of the ectocuneiforni. This ligament is homologous with the dorsal astragalo- 

 scaphoid and scapho-cuneiform ligaments of man. The tubercle in question is present in both Lemur 

 and Nothardus. The ]50steroinferior pai't of the head, al)ove the navicular facet and below the sus- 

 tentacular facet, articulates with, and rests uj^on, the Inroad tibio-navicular ligament. 



The astragalus of N'otluirdus differs from that of Lcnmr in the following details: the neck'is relatively 

 longer, the trochlea narrower, the facet for the fibula is more concave, the concavity of the ectal facet is 

 more pronounced, the pit for the posterior astragalo-fibular ligament is deeper. But these are all minor 

 differences of no importance in comparison with the fundamental agreements observable in every part 

 of the bone. 



To sum up with regai-d to the function of the astragalus in iVotlKirctus and in later jjriniatcs, this 

 bone transmits the weight of the body from the tibia and fibula above to the calcaneum and navicular 

 below. In leaping, on the contrary, the astragalus transmits the thrusts from these elements upward. 

 The joint at the trochlea is hinge-like and permits motion chiefly in the plane of the tibia. Tlie cross- 

 ing of the astragalus and calcaneum, so that the neck of the astragalus is sharply inclined from without 

 inward and downward, seems to be connected chiefly with the inverted position of the pes, the sole of 

 the foot being appressed to the outer side of the supporting trunk or branch and the hallux, on the inner 

 side, raised on a higher level ; as a result of this the navicular through the pressure of the entocuneiform 

 stands on higher level than the cuboid, so that it carries with it the head of the astragalus, the neck of 

 the astragalus consequently assuming an oblique position. In specialized cursorial animals, on the 

 other hand, the crossing of the astragalus upon the calcaneum is less pronounced, and the neck of the 

 astragalus tends to get in line with the tibia. 



For similar reasons the head of the astragalus is twisted around toward the inner side, that is l^ecause 

 the whole inner side of the tarsus is twisted toward the inner side along with the hallux. The rocking 

 of the astragalus upon the calcaneum, and the cog-tooth relation of the ectal astragalo-calcaneal facets 

 have been mentioned above, as well as the axle-like function of the posterior astragalo-fibular and the 

 posterior astragalo-tibial ligaments. The ball-like head of the astragalus permits a wide degree of flexion 

 of the foot and considerable twisting of the navicular upon the astragalus. 



Such were the chief morphological and functional characteristics of the astragalus of these relatively 

 primitive Eocene primates. Together with other evidence, they indicate a long course of arboreal adap- 

 tation preceding the lemuroid stage. 



