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Genus FJSSA. 



Larus apud Brisson, Orn. vi. p. 185 (1760). 



Gavia apud Boie, Isis, 1822, p. 563. 



Rissa, Stephens in Shaw's Gen. Zool. xiii. pt. 1, p. 181 (1825). 



Cheimonia apud Kaup, Natiirl. Syst. p. 84 (1829). 



Laroides apud C. L. Brehm, Vog. Deutschl. p. 754 (1831). 



This genus contains only two species : — Rissa tridactyla, which inhabits the Palsearctic, Ethiopian, 

 and Nearctic Kegions ; and Rissa brevirostris, which inhabits the North Pacific Ocean from Alaska 

 to Kamtschatka. These birds differ from the species included in the genus Larus either in having 

 the hind toe rudimentary or else in lacking it entirely. In habits they do not vary from the 

 true Gulls ; and, like them, they build a tolerably bulky nest of seaweeds and grass, which they 

 place on the cliffs, and deposit three eggs, which vary considerably in colour and markings, the 

 ground-colour being greenish white, pale ochreous, or light olivaceous, spotted, and the markings 

 dark brown and purplish grey. 



Rissa tridactyla, the type of the genus, has the bill rather short, moderately stout, nearly 

 straight, the upper mandible decurved towards the tip, the lower mandible narrower, compressed, 

 the intercrural space long and narrow, the crura erect, convex, their lower outline slightly 

 concave, forming a slight prominence at the commissure ; nostrils median, linear, oblong, covered 

 above and behind with a sloping, convex, thin-edged plate ; wings long, pointed, the first quill 

 longest, the second nearly as long ; tail moderately long, even ; legs short, the tibia bare for a 

 short distance, tarsus scutellate ; hind toe obsolete, anterior toes moderately long, the inter- 

 digital membranes slightly emarginate ; claws moderate, compressed, slightly curved, rather 

 acute, that on the middle toe with the inner edge slightly dilated. 



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