LARUS. 297 



The variations in size of the Glaucous and Iceland Gulls are so great that 

 in a large series it is sometimes very difficult to dravr the exact line 

 where one begins and the other ends ; but exceptionally large examples of 

 the Iceland Gull and exceptionally small examples of the Glaucous Gull are 

 so rarely met with that the difficulty of discriminating between the two 

 species is more imaginary than real ; possibly they may be only sub- 

 specifically distinct. 



a. Tail forked L. sabinii. 



b. Tail strong-ly cuueifonn L. iiossi. 



c. Tail even. 



a\ Length of Aviiig under 9| inches L. minutus. 



b^. Length of wing from 10 to 15 inches. 



«". Primaries brown, with white terminal or subterminal spot. L. canus. 

 b^. Primaries French grey, broadly tipped with black ; no 



hind toe L. tridactylus. 



C-. Primaries white, with or without a brown terminal spot. L. ebukneus. 

 d-. Primaries white, with black tips, always margined on the 



inner web with black, and generally on outer web. ... L. ridibundus. 



e^. Primaries white, with black tips and outer webs L. Philadelphia. 



c^. "Wing over 15 inches long. 



f~. Primaries nearly uniform white or pale brown. 



«'. Wing less than 17 inches long L. leucopterus. 



b^. Wing more than 16| inches long L. glaucus. 



ff"^. Primaries nearly uniform dark brown; wing less than 



16 inches. Mantle of adult dark slate-grey L. fuscus*. 



h^. A pale wedge (sharply defined at the tip) on the inner 

 web of the second, third, and fourth primaries ; length 

 of wing from lof to 17 h inches. Mantle of adult pale 



French grey L. argentatus. 



1^. A distinct wedge on the third and fourth primaries ; 

 length of wing from 17^ to 19^ inches. Mantle of 

 adidt dark slate-grey L. marinus. 



* It is probable that when this group of Gulls is investigated a complete series wiU be 

 found connecting L. fuscus, through L. affinis, and L. cachinnans with L. aryentatus. 

 Special attention shoidd be directed to the possible occurrence of L. affinis in our islands, 

 as it has more than once been obtained in Heligoland. It is said to differ from L. fuscus 

 in having a paler mantle, and in having the pattern on the primaries more clearly defined, 

 and in very old birds appearing on the first primary, as in L, argentatus. 



