416 THE OSTEOLOGY OF VULPES MACROTIS. 



sponding to the curve of the internal trochlea of the superior aspect. When 

 normally luxated, the astragalus rests upon the supero-internal aspect of the os 

 calcis and articulates with it by two facets. Above, it articulates with the tibia 

 and fibula ; the former monopolizing all of the trochlear surface, while the infero- 

 internal aspect of the distal end of the latter articulates with the greatly curved 

 and narrow facet, described above as composing the outer surface of the tarsal bone 

 under consideration. 



The os calcis or calcaneum has an extreme length longitudinally of 2.4 cms., 

 and a width of one centimetre, measured at its widest part across the articulation 

 for the astragalus. This latter occupies the anterior half of the bone superiorly, 

 while its anterior face is occupied by a vertical facet devoted to the articulation 

 with the cuboid. The posterior half of the calcaneum is rather deep, com- 

 pressed from side to side, with the free upper extremity enlarged, and bears a 

 smooth, vertically convexed, laterally concaved surface for the insertion of the 

 distal end of the tendo Achillis. The " lesser process" of the os calcis is well 

 developed, and beneath it is plainly to be seen the groove for the passage of the 

 tendon of the flexor longus pollicis muscle. Flower says 10 that the recent researches 

 of Baur, On the Morphology of the Tarsus in the Mammals, "American Naturalist," 

 January, 1885, make it probable that a certain bone on the tibial side of the tarsus 

 of Hyrax, many Edentates, Ornithorhynchus and rodents, — and hitherto looked 

 upon as a sesamoid bone, — is the rudimentary tarsale tibiale, whilst the astragalus 

 is the intermedium representing the lunare of the hand. This tibiale is frequently 

 fused with the centrale, the navicular bone in such cases containing the elements 

 of the centrale and tibiale." Again, in the same work, on page 342, he says, " In 

 addition to these constant tarsal bones, there may be supplemental or sesamoid 

 bones ; one situated near the middle of the tibial side of the tarsus, largely 

 developed in many carnivora and rodents ; another, less frequent, on the fibular 

 side; and a third often developed in the tendons of the plantar surface of the 

 tarsus, especially large in the armadillos. There is also usually a pair of sesa- 

 moid bones opposite each metatarso-phalangeal articulation on its plantar aspect." 

 In carefully examining these parts in Vulpes macrotis it becomes clear that the 

 first-mentioned bone (or sesamoid ?), considered in the above footnote to be the 

 tibiale, is fused with the centrale or navicular bone on its plantar side. The smaller 

 one, on the fibular side, if present at all, must be either extremely rudimentary or 

 else co-ossified with some other tarsal element. It was not found in this fox. In a 

 former paragraph, the paired sesamoids on the plantar aspect of each metatarso- 

 phalangeal articulation, have already been described. 



Passing to the cuboid we meet with a large strong bone having a length equal 

 to that of the scaphoid and external cuneiform combined. It articulates with both 

 of these bones as well as with the proximal extremities of the fourth and fifth 

 metatarsals anteriorly and the os calcis behind. Dorsally, the bone is comparatively 

 smooth, while on its ventral side in front it presents a deep groove, which crossing 

 of the Mammalia, Third edition, p. 340. 



