96 FIELD MUSEUM OF NATURAL HISTORY ZOOLOGY, VOL. XIV. 



from the head. The inner malleolar facet is extensive and sharply 

 deflected from the medial tibial facet from which it is separated by a 

 high trenchant ridge which extends over the neck to the head at the 

 navicular facet. In all other forms examined, this ridge is discontinuous 

 or confined to the trochlea. The formation of a depression in this ridge 

 between the trochlea and the head would seem to be the first step in 

 the separation of the head from the trochlea. Hence, in this respect the 

 astragalus of Canolestes may be primitive. The tibial facet is very 

 slightly concave and merges imperceptibly with the fibular facet which 

 is narrow and considerably deflected. The sharp division of the two 

 principal tibial facets virtually permits the tibia and astragalus to be 

 locked together almost as effectively as in the macropods. Lateral 

 movement of the tibia is practically precluded and the general freedom 

 of the ankle joint found in most polyprotodonts is quite impossible. 

 Even such forms as Sminihopsis and Antechinomys have much looser 

 ankle joints than Ccenolestes. 



The inferior surface of the astragalus in C&nolestes is largely occupied 

 by the sustentacular facet which is unusually broad and convex. It 

 is nearly at right angles but confluent with the large navicular facet 

 which covers the head of the bone and runs up the inner side for a 

 considerable distance. The ectal facet is crescentic in outline and is 

 separated from the sustentacular by a deep groove. The inner anterior 

 margin of the navicular facet has slight contact with the cuboid bone 

 but there is no distinct development of a cuboid facet. A small sesamoid 

 is situated between the astragalus and the posterior distal extremities of 

 the tibia and fibula. 



The os calcis has the tuber laterally compressed and very slightly 

 flexed downward and inward. The anterior sustentacular part of the 

 bone is relatively large and broad. In addition to the usual ectal, 

 sustentacular, and cuboid facets it has a prominent triangular projec- 

 tion on its antero-external surface for the attachment of the tarso- 

 metatarsal and perhaps other ligaments. 



The navicular is somewhat anvil-shaped with a smaller dorsal head 

 articulating with the astragalus and slightly constricted from the larger 

 plantar part which spreads inward to the middle of the sole, its plantar 

 surface being broad and flat instead of narrow and ridgelike. Distally 

 it articulates with the ectocuneiform, entocuneiform and mesocuneiform. 

 The cuboid articulates distally with the two outer metatarsals and with 

 the proximo-external surface of the mesocuneiform. Its plantar surface 

 has the usual groove for the tendon of the peroneus longus muscle. The 

 ectocuneiform, entocuneiform and mesocuneiform are rhomboidal bones, 

 the first supporting the hallux, and the others the second and third 



