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innermost is the most produced, the outermost the least. The articular surface resem- 

 bles in character that on the end of the third metatarsal, but is relatively smaller ; it 

 consists of a longitudinal median prominence, continuous with a smooth narrow tract on 

 the inner side, on which surface articular cartilage and synovial membrane seem to have 

 existed at only a small part at its lower end. On the outer side, the smooth tract is 

 limited to the upper half of the prominence ; this is slightly concave vertically or length- 

 wise. It is a form of articulation calculated rather for firm and unyielding junction 

 than for flexibility. 



The outermost metatarsal (Plate XXVI. fig. 1, m s), answering to the fifth of the 

 pentadactyle foot, is the longest, but from its more backward articulation with the 

 tarsus it does not reach so far forward as the fourth. 



The proximal end is a free rough tuberosity, somewhat more than an inch anterior 

 to which, on the tibial side, is the oval slightly concave articular surface adapted to the 

 cuboides (Plate XXV. fig. 2, v), which surface is continuous with the short transverse 

 flattened surface for the fourth metatarsal ; and, in advance of this, is the larger, sub- 

 circular, slightly undulated surface for the corresponding surface on the fourth meta- 

 tarsal. The second surface is on the middle of the inner surface of the fifth metatarsal, 

 and is bordered in front and beneath by the very rough honeycombed surface for 

 ligamentous junction with the adjoining metatarsal. This combination of synovial with 

 syndesmotic joints admits of that degree of slight elastic movement of the very firmly 

 attached fourth and fifth metatarsals, which must have facilitated the heavy tread of the 

 ponderous quadruped, and have alleviated the effect of the enormous pressure on the 

 two gradatorial toes. The whole outer and under part of the non-articular surface of 

 the fifth metatarsal is strongly sculptured by irregular ridges, tubercles, grooves, and 

 foramina, indicative of the hoof-like callosity of the outer border of the sole in which it 

 was chiefly imbedded. The upper and inner non-articular surfaces are comparatively 

 smooth. The inner side is flat, and is traversed by an oblique shallow (tendinal 1) groove. 

 On the distal surface are two small articular facets ; the inner one, which is the best 

 marked, is subcircular, about 8 lines in diameter. 



Phalanges. — The bones of this class are unusually reduced in number in the foot of 

 the Megatherium, even admitting the accuracy of the figures of the hind foot of the 

 Madrid skeleton, in which two stunted phalanges appear to terminate both the fourth 

 and fifth toes. For, as the great unguiculate toe, like that of the fore-foot, has only 

 two moveable phalanges, the total number of these bones is but six, not exceeding that 

 of the tarsal bones of the same foot. 



The phalanx (Plate XXVI. figs. 1 & 3, i * a ) of the innermost toe (Hi), answering to the 

 third of the pentadactyle foot, represents, as in the corresponding digit of the fore foot, 

 the proximal and middle phalanges connate. The compound bone (ib. fig. 3, i & i) is 

 shaped like a wedge with an oblique edge, deeply notched. It has articular surfaces 

 not only on its proximal and distal ends, but upon its upper or dorsal surface. The two 

 former surfaces, which are the ' ends ' of the bone in ordinary phalanges, here form the 



