412 



ANATOMY OF VERTEBRATES. 



2S0 



proximal end, including the olecranon which is twice as broad as 

 long, and projects backward rather than upward. The proximal 

 end of the radius, 54, is circular, and its articular modifications 

 are as well adapted for rotatory and flexile movements of the 

 antlbrachial bones as in the human arm. The interosseous space 

 is shorter and much narrower relatively than in the Sloths. Of 

 these the Ai, fig. 280, has the carpus reduced to six bones, the 

 scaphoid being connate with the trapezium, s t, and the mag- 

 num with the trapezoides, m. A rudiment of the metacarpal 



of the pollex, i, has co- 

 alesced with that of the 

 index, ii, and a rudiment 

 of the metacarpus of the 

 minimus with that of the 

 annularis, x, iv. In the 

 three functional digits the 

 proximal and middle pha- 

 lano;es are confluent, 1...2 : 

 the unequal ones are of 

 great length, and restrict- 

 ed in their movements, by 

 the production of the back 

 part of their base, to de- 

 grees of flexion. The joints of all the digits are deeply trochlear. 

 The bones of the hand of the Unau (Choloepus) are described 

 at p. 306, fig. 191, '^ Sloth.' In the Megathere the wrist has a 

 scapho-trapezium, fig. 191, st, but the trapezoid and magnum 

 are distinct. The pollex is represented by a stunted meta- 

 carpal, I: that of the minimus, figs. 191, 279, v, is long and 

 supports one or two short thick phalanges : the second, 11, 

 third. III, and fourth, iv, digits are powerfully unguiculate, 

 but the first and second phalanges coalesce only in the medius, 

 1, 2, III. The massive metacarpal is squared, firmly attached to 

 the contiguous ones, with the outer angle of the base produced and 

 wedged betAveen that of the annularis, the magnum, and unciforme.^ 

 In the Sloths the femur, fig. 264, 65, is straight, like the 

 humerus, but is thicker and shorter ; the head shows no impres- 

 sion for a ligamentum teres. The tibia, 66, and fibula, 67, are 

 opj)ositely bent, leaving a mde interosseous space, as in the fore- 

 arm, but are still shorter than their homotypes. The inner 

 malleolus projects backward and supports a grooved process : the 

 lower end of the fibula fits, like a pivot, into a socket in the 

 ' xcr. p. 53, pi. xxi. 



Bones of fore-foot, Ai 



