772 U. S. p. R. E, EXP. AKD SURVEYS — ZOOLOGY — GENERAL REPORT. 



Sub-Familj ANATINAE. 



The Anatinae, or river ducks, are easily known by their having the tarsi transversely scutel- 

 late anteriorly, and the membrane or lobe of the hind toe narrow and much restricted. The 

 legs are longer than in Fuligulinae, but shorter tlian in the geese. 



The differences in external form of the American river or fresh water ducks are very trifling, 

 excepting in Spatula, and are scarcely of the generic value allotted them by ornithologists. 

 The system of coloration would seem to furnish as reliable a basis for subdivisions as the form, 

 and it is by this, in great measure, that the genera are determined. 



All the North American river ducks agree in having the crissum black. In all, excepting 

 Querquedula, there is a tendency to waved lines on the feathers of the flanks, most conspicuous 

 in the mallard, gadwall, and green winged teal. 



Synopsis of genera. 



A. Bill rather longer than the foot; the sides nearly parallel; lamellae scarcely visible in the 

 lateral profile, except in Querquedula. Tail about two-fifths the wing, except in Dafila. 



Anas. — Bill broad and the edges parallel ; the width more than about one-third the 



lower edge. Tail two-fifths the wing. 

 Dafila. — Bill narrow, widening somewhat to the tip. The width less than one-third 



the commissural or lower edge. Tail much pointed, three-fourths or more the wing. 

 Nettion. — Bill very narrow, the sides parallel ; the width scarcely more than one-fourth 



the lower edge. Nail very narrow, the width one-fifth that of the bill ; upper angle 



of the bill not reaching as far back as the beginning of its lower edge. 

 Qderquedula. — Width of bill about one-third the length of lower edge, or rather more. 



The nail about one-third the width of bill. Upper angle of the side of the bill 



extending rather further back than the lower edge. Lamellae distinctly visible in the 



lateral profile. 



B. Bill much longer than the foot, widening towards the end to double the breadth at the 

 base. 



Spatula. — Lamellae much developed, projecting downwards much below the edge of the 

 bill. 



C. Bill not longer than the head, and shorter than the foot. The upper posterior angle on 

 the side of the bill obtuse, and not extending backwards as far as the lower edge. Tail about 

 two-fifths the wing. 



Chaulelasmos. — Bill as long as the head ; the lower edge about as long as the outer toe, 

 and longer than the tarsus. The lamellae distinctly visible below the edge of the bill 



Mareca. — Bill shorter than the head ; the lower edge about equal to the tarsus, and to 

 the inner toe. The feathers at base of bill above extending across nearly in straight 

 line. 



D. Bill shorter than the head, and elevated at the base ; the upper lateral angle extending 

 backwards and upwards considerably behind the lower edge. 



Aix. — Nail very large and much hooked, forming the tip of bill. Nostrils very large, 

 the feathers of forehead reaching to the posterior edge. Tail half the wings. 



