° 1915 J Johnson, A Four-winged Wild Duck. 471 



from the under side of the former at the region of the elbow, pre- 

 senting corresponding surfaces and with divisions of forearm and 

 hand clearly indicated. The feathery covering shows no modifi- 

 cations representing flight feathers but consists of under wing- 

 coverts which belong primarily to the feather tracts of the normal 

 pair. The broadly white-tipped posterior series of under wing- 

 coverts of the primary wing continues onto the posterior margin 

 of the supernumerary appendage while the rest of the latter is 

 covered with the smaller, darker feathers of the anterior series. 



The accessor}' wing of each side feels rigid at the elbow and has 

 no movement independent of the primary wing. It is partly flexed 

 at the point corresponding to the carpal region and here it can be 

 felt that a slight movement is possible, but apparently complete 

 flexion or extension can not take place. When the primary wings 

 are folded in place against the body the tips of the smaller set pro- 

 ject beyond their margins ventrally as a pair of inconspicuous 

 feather tufts. The right projects a trifle further, and the integu- 

 ment covering its tip is scarred. The accessory wings may possibly 

 during life have interfered somewhat with the folding of the larger 

 pair though in the dead bird this is not apparent. 



Skeleton. It is evident that in an abnormality like the present 

 case any attempt to speak of homologies must result more or less 

 unsatisfactorily. This applies to the bony parts as well as to the 

 muscles, and while in the following account the supernumerary 

 parts may be referred to in terms of normal structures it is not in- 

 tended to convey the impression that homologies in any strict 

 sense exist. 



No abnormal features were found in the shoulder girdle. On 

 the two sides the bony elements of the accessory wings are essen- 

 tially alike from the elbow joint distally but the upperarm portions 

 present markedly different conditions. 



On the left side (Fig. 5) the distal end of the humerus of the pri- 

 mary wing is shattered. The remaining part of the bone is of 

 normal shape. On the inner aspect of this bone, at the junction 

 of the shaft with the head is a slender process of bone 7 mm. 

 in length, extending roughly parallel with the shaft of the humerus. 

 At its distal end the process passes into a slender, cylindrical, tendi- 

 nous ligament 15 mm. in length, which continues toward the 



