AVES. 



257 



form the wrist or carpus (<?), but these are reduced to two in 

 number. The carpus is followed by the bones which con- 

 stitute the root of the hand or metacarpus (m), but these are 

 also reduced to two, instead of being five in number, as they 

 are in most Mammals. The two metacarpal bones are also 

 amalgamated together at both ends, so as to form a single 

 piece, at the base of which, on its outer side, is a rudimentary 

 digit, the "thumb" (), which carries a tuft of feathers, known 

 as the " bastard wing." The metacarpal bones, finally, sup- 

 port each a single finger (p), of which one is never composed 

 of more than one bone or phalanx, while the other is com- 

 posed of two or three phalanges. (To understand thoroughly 

 the leading modifications of the limbs of birds, the student 

 will do well to refer to the general description of the limbs of 

 Vertebrates, p. 200, Figs. 96, 97.) 



As regards the composition of the hind-limb in birds, the 

 two halves of the pelvic arch (i. e., the innominate bones) al- 



FIG. 129. A, Pelvis and bones of the Leg of the Loon or Diver (after Owen) : i Innominate 

 bone ; / Thigh-bone (femwr) ; t Tibia, r fibula, together forming the shank ; m Tarso- 

 metatarsus ; p Phalanges of the toes. B, Tail of the Golden Eagle ; a Ploughshare- 

 shaped bone, carrying the great tail-feathers. 



