° i9i5 J Johnson, A Four-ivinged Wild Duck. 475 



To the outer side of this muscle is a spindle-shaped muscle 

 (Fig. 4, 2) originating by a relatively long, narrow and flattened 

 tendon from the outer, posterior surface of the anchylosis and pass- 

 ing distally, obliquely across the anterior surface of the forearm 

 bone, to become inserted also by a relatively long, slender tendon 

 on the anterior, inner surface of the proximal end of the metacarpal 

 bone. The relations of this muscle closely approach those of the 

 M. extensor metacarpi radialis longior of the normal wing, but 

 the two well defined heads of the latter are here lacking. This 

 muscle would have a pronating action upon the metacarpus in 

 addition to the extending function. It is to be noted that the inner- 

 vation of this muscle is by a branch from the N. brachialis longus 

 inferior, while the M. extensor metacarpi radialis longior in the 

 normal wing is supplied by the Nervus radialis. 



On the under or medial surface of the anchylosed area there arises, 

 partly by fleshy fibers and partly by a flattened tendon, a muscle 

 mass which further distally is differentiated into two muscles, each 

 with a long, slender tendon. One of these components (Figs. 3 and 4, 

 3) is proximal, and its tendon which is much the longer, passes to 

 the under side of the wrist where it is held in place by a fibrous 

 sheath, and thence courses along the under surface of the metacarpal 

 bone to become inserted at the base of the phalanx. The other, 

 more distal muscle becomes inserted into the fibrous capsule of the 

 wrist joint, on its under side and anteriorly, where its tendon is 

 held in place by the tendon of muscle 2. The first of these 

 muscles has an insertion corresponding rather closely to the M. 

 flexor profundus digitorum, the second to the M. flexor carpi ulnaris 

 brevior of normal wings. 



On the ulnar side of the under surface of the forearm is a super- 

 ficial, broad, thickened, tendinous sheath (Figs. 3 and 4, T. s.). This 

 sheath encloses the elbow joint of the supernumerary wing proxi- 

 mally, and about the middle of the forearm it separates into two 

 bands which diverge, one passing to the outer side of the carpal 

 joint where it inserts, and the other, a narrower band, passing to its 

 insertion on the inner side of the joint. This tendinous sheath 

 encloses a comparatively stout muscle, 5, which is exposed in its 

 distal half by the division of the sheath. The muscle originates on 

 the inner epicondylar region of the rudimentary upperarm bone by 



