100 



GREGORY: NOTIIARCTUS, AN AMERICAN EOCENE PRIMATE 



of which the inner two, for the ento- and mesocuneiform respectively, are convex, the outer concave. 

 The middle one, however, is more convex than the homologous facet in Lemur and is not displaced pos- 

 teriorly but is in line Avith the other two, these differences being correlated with certain differences in 

 the mesocuneiform to be noted below. 



Ectocuneiform 

 Plate XXX 



The ectocuneiform forms the keystone of the transverse arch of the instep. It is compressed later- 

 ally between the mesocuneiform internally and the cuboid externally, and it is extended vertically (or 

 anteroposteriorly when the foot is flat on the ground). It receives the oblique transverse thrusts from 

 the hallux, which are transmitted through the mesocuneiform and through the upper end of metatarsal 

 II, and in turn it distributes these thrusts to the navicular, cuboid and digits III and IV. It also trans- 

 mits proximodistal thrusts from the navicular to digits III and IV and vice versa. 



On the dorsum of the foot the ectocuneiform is decidedly narrower at the upper or proximal end than 

 it is at the lower or distal end. The narrow upper end bears a slightly concave facet for the anterior 

 pedicle, or dorsolateral facet, of the navicular. The wide lower end bears a concave triangular facet 

 for metatarsal IV. The inner or flat side bears near the upper end a proximodistally extended, slightly 

 concave, narrow facet for the mesocuneiform, and near the lower end a flat facet for metatarsal II. The 

 outer or lateral side, as seen on the dorsum of the foot, is concave near the upper end and bears a flat 

 upper facet for the cuboid. The sinus, or concavity, on this outer side is partly filled by the cubo-ecto- 

 cuneiform ligament; below, or distally, the lateral side bears a concave lower facet for the cuboid. 



The plantar aspect of the bone on the under side of the tarsus is very narrow, while the sides of the 

 bone beneath the dorsum and facing the mesocuneiform and the cuboid respectively are, as it were, pressed 

 in towards each other. 



The deep portion of the up])er or proximal end, deep to the convex facet for the na\'icular already 

 mentioned, ends proximally in a narrow keel bearing on its outer or lateral side a facet for the deep surface 

 of the cuboid, and on its inner side a flat facet for the deep portion of the mesocuneiform. Thus the 

 deep portions of the cuboid and of the mesocuneiform arch over the deep upper end of the ectocuneiform 

 and are in contact above it. This is a result of the lateral folding of the foot so that the plantar surface 

 of the hallux inclines toward those of the other digits. To the prominent tubercle, which is immediately 

 below or distal to the above described keel, and which is compressed laterally and extended proximo- 

 distally, may have been attached a transverse ligament, running from the entocuneiform to the meso- 

 and ectocuneiforms, and a proximodistal ligament running down to the back of metatarsal III. 



The foregoing description of the ectocuneiform of Notharctus also applies word for word to that of 

 Le7nur. Nevertheless, there are a few quite minor differences that distinguish the ectocuneiforms of 

 these two genera. In Lemur the dorsal aspect of the bone is more rectangular and the sinus that sepa- 

 rates it from the cuboid is less conspicuous. The proximal facet for the navicular is wider and the tubercle 

 on the plantar surface is much less prominent. The facet for metatarsal II on the inner side of the bone 

 does not extend nearly so far up toward the proximal end of the bone, as metatarsal III is not thrust 

 upward so far between the ento- and the ectocuneiform. 



