AVES. 77 



three anterior toes united ; hind toe free, some- 

 times represented by a simple nail, or altogether 

 wanting 1 . 



108. STERNA. Bill as long as, or a little longer 

 than the head, nearly straight, rather compressed, slender, 

 attenuated towards the tip, sharp-pointed ; mandibles of 

 equal length, the upper one slightly bending at the tip, 

 lower one with the mental angle a little projecting : nostrils 

 longitudinal, oblong-linear, pervious : feet weak, small, four- 

 toed ; membranes deeply notched : wings long, acuminated ; 

 first quill longest : tail generally forked. 



(1. STERNA.) Wings very long: tail deeply forked. 

 (2. ANGUS.) Wings moderate : tail even. 



109. LARUS Bill moderate, strong, cultrated, much 



compressed ; the .culmen continuous, bending downwards at 

 the tip ; gonys obliquely ascending, straight ; mental angle 

 strongly projecting ; tomia sharp : nostrils longitudinal, 

 medial, oblong-linear, pervious : legs moderate : anterior 

 toes with the membranes entire; hind toe small, high on 

 the tarsus, rarely wanting : wings long ; first or second 

 quill longest : tail generally even. 



(1. XEMA.) Tail slightly forked. 

 (2. LARUS.) Tail even. 



110. LESTRIS Bill moderate, strong, thick, rounded 



above, compressed towards the tip ; culmen and dertrum 

 distinct, the latter convex and strongly hooked ; gonys 

 straight, obliquely ascending ; mental angle projecting : 

 nostrils immediately behind the dertrum, diagonal, nar- 

 row, closed behind, pervious : hind toe very small, nearly 

 on a level with the others : wings with the first quill long- 

 est : two middle tail feathers elongated. 



111. PROCELL ARIA. Bill more or less elongated, 

 straight, subcylindrical, depressed and dilated at the base; 

 culmen and dertrum distinct; the latter convex, much com- 

 pressed, and strongly hooked ; lower mandible with a longi- 



