106 



oregory: notharctus, an American eocene primate 



below. Distally it bears a spreading asymmetrical expansion which is produced posteromedially into a 

 prominent process. 



The plantar surface of this distal end bears a facet for the distal phalanx and for the large sesamoid 

 of the joint capsule. This facet is convex' distally, for the distal phalanx, and concavoconvex on the 

 plantar aspect, for the sesamoid. The sesamoid was developed in the tendon of the powerful adductor 

 transversus muscle. 



The asymmetrical distal phalanx spreads widely at the proximal end, its anterolateral process being 

 much the larger of the two. Onto its flattened plantar surface was inserted the tendon of the flexor 

 longus hallucis. The distal end was spade-shaped and bore a large flat nail. 



The foregoing description of the hallux of Notharctus would apply literally to that of Lemur, the chief 

 difference being that in the modern genus the distal phalanx is more expanded. Lepilemur has the distal 

 phalanx and nail still more expanded. The Propithecus hallux is very similar to that of Notharctus, 

 but there are minor differences in proportion, the metatarsal in Propithecus being long. 



The remaining digits of the pes of Notharctus, so far as known, require only brief notice, the chief 

 differences from Lemur being the shortness of the metatarsals, the length and slenderness of the phalanges 

 and the narrowness of the distal phalanges; the second metatarsal is not thrust up into the tarsus as far 

 as it is in Lemur. The respective lengths of the digits are not positively known, but digit IV was probably 

 the longest as in lemurs. 



From the foregoing description and accompanying illustrations it is evident that the astragalus, 

 the calcaneum, the navicular and the entocuneiform of A^otharctus differ only in minor details and pro- 

 portions from those of Lemur; the mesocuneiform is broader and more vertical in position and the ecto- 

 cuneiform is narrower at its upper end. The cuboid is closely similar to that of Lemur, and likewise 

 exhibits on its outer side a groove for the tendon of the peroneus longus. The hallux is not quite so 

 divergent as that of Lemur, the metatarsals are shorter and much stouter; all the phalanges of digits 

 II-V are more slender and narrow. 



The pes of Notharctus is somewhat less specialized than that of any modern lemur. It is obviously 

 also much more primitive than that of Propithecus, the chief specializations of which are the massive char- 

 acter of the tarsals and the great elongation of the metatarsals. The hallux of this genus is essentially 

 similar to that of Notharctus. E\'idently in Propithecus as well as in Notharctus the adductor hallucis, 

 adductor obliquus hallucis and adductor transversus hallucis were very powerful, as indicated by the 

 large size of the phalanges of the hallux. The internal process on the proximal end of metatarsal I is 

 even heavier in Propithecus than it is in Notharctus; this process abuts against the mesocuneiform and 

 metatarsal II when the digit is adducted; it interlocks with the surface of the cuneiform and metatarsal 

 II, and offers a firm attachment for the tendon of the peroneus longus. This muscle was doubtless very 

 effective in pressing the hallux and its pads against the side of the branches. 



The pes of Notharctus differs in many important details from that of Cebus, the latter being less 

 pincer-like and with a more mobile hallux, less widely divergent. The hallux of Cehus is held more 

 parallel to the other digits than that of Notharctus; in correlation with this, the distal facet of the ento- 

 cuneiform is less saddle-shaped and more convex and faces distally rather than medially. The ento- 

 cuneiform of Cehus lacks the prominent posterior process seen in Notharctus. The metatarsal of the 

 hallux in Cehus has the peroneal process much reduced; while the ectocuneiform is much shorter verti- 

 cally. The cuboid is more rectangular; the groove on the back of the cuboid for the tendon of the long 

 pei'oneal muscle is much broader and deeper. The astragalus has a more delicate neck and a wider 



