CEYLON RAILS, WADERS, GULLS, AND TERNS. 245 
toes, and the character of the bill. Of the four genera, two— 
Hydroprogne and Anous—each give us a single species ; 
Hydrochelidon furnishes us with two, one of which is common 
on inland waters, the other rare. Of the genus Sterna, we 
have no less than twelve species. Two of these can be 
distinguished at once by the colour of their upper plumage, 
which is sooty-brown instead of some shade of pearly-gray ; 
the remaining ten have to be differentiated mainly by their 
wing measurements and the colour of their bills and feet. In 
consequence the key may not seem very simple in its working, 
but it should be borne in mind that the gray-mantled. species 
usually met with are either the three largest or the two 
smallest. Of the five medium-sized species, only one—S. 
dougallimis in any degree common, the other four are rare, 
two being rather doubtful inclusions in the Ceylon list. 
Rough Key to Ceylon Sternine. 
I.— Medium size ; wing 8 to 9 inches ; webs deeply scalloped 
between the toes ; tail short and only slightly forked. Genus 
Hydrochelidon. 
(a) Bill from gape 1:6; in winter the black spots on the 
crown extend forward of the eyes ; in summer the 
wing lining remains white. 
Hydrochelidon hybrida (the Whiskered Tern). 
(6) Bill from gape 1°3; in winter the black spots on the 
crown do not generally extend forward of the eyes ; 
in summer wing lining turns black. 
Hydrochelidon leucoptera (White-winged Black 
Tern). 
II.—Tail forked, but very short; bill powerful; webs 
fully developed ; size large. Genus Hydroprogne. 
(a) Wing 15°5; bill red. 
Hydroprogne caspia (Caspian Tern). 
Il1.—Tail deeply forked, the outer pair of feathers notice- 
ably longer than the rest; bill fairly slender; webs well 
developed. Genus Sterna. 
