504 THE VERTEBRATE SKELETON. 



union of carpalia, 4 and 5 to form the unciform bone. Two 

 sesamoid bones are commonly developed, one on each side of 

 the carpus, the pisiform or one on the ulnar side being much 

 the larger and more constant : it has been suggested that 

 these represent respectively vestiges of a prepollex and a post- 

 minimus digit 1 . 



One or more of the five digits commonly present may be 

 lost, and sometimes all are lost except the third. The terminal 

 or ungual phalanges of the digits are commonly specially 

 modified to support nails, claws, or hoofs. There are as a rule 

 two small sesamoid bones developed on the ventral or flexor 

 side of the metacarpo-phalangeal articulations, and sometimes 

 similar bones occur on the dorsal or extensor side. 



MONOTREMATA. In Echidna the carpus is broad, the sca- 

 phoid and lunar are united and there is no centrale. The 

 pisiform is large and several other sesamoid bones occur. Each 

 of the five digits is terminated by a large ungual phalanx. In 

 OrnithorhyncTius the man us is more slender, but the general 

 arrangement is the same as in Echidna. 



MARSUPIALIA. The carpus has no centrale and the lunar 

 is generally small or absent. Five digits are almost always 

 present. In Choeropus however the only two functional digits 

 are the second and third, which have very long closely apposed 

 metacarpals ; the fourth digit is vestigial, but has the normal 

 number of phalanges, while the first and fifth are absent. The 

 manus in Notoryctes is extraordinarily modified, the scaphoid 

 and all the distal carpalia are apparently fused, the first, 

 second, and fifth digits are very small, the third and fourth, 

 though having only one phalanx apiece, bear each an enormous 

 claw. Lying on and obscuring the ventral surface of the manus 

 is a large bone, probably a sesamoid. 



Among the EDENTATA there is a great diversity in the 

 structure of the manus, the centrale is however always want- 

 ing, and except in Manis the scaphoid and lunar are distinct. 

 1 See K. Bardeleben, P. Z. S., 1889, p. 259. 



