NATURAL SCIENCES OF PHILADELPHIA. 
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forked. Legs rather .stout ; hind toe sometimes rudimental. Head never 
with a hood ; in winter with the neck streaked with dusky. Under parts 
white without a decided roseate tint. 
1. Larus . — Size large or moderate. Bill stout, robust, obtuse, the tip not 
attenuated, the angle usually very prominent. Convexity of culmen great 
at the ends. Color white, nearly always with a darker mantle. Tail even. 
2. Blasipus . — Size moderate. Bill rather slender, its tip somewhat attenu- 
ated. General color dusky. Tail even, or very slightly emarginate. 
3. Rissa . — Size rather small. Bill stout at base, but more attenuated and 
decurved at the tip. Angle acute, but not very prominent. Hind toe rudi- 
mental. Tail even ; somewhat emarginate in the young. 
4. PagophilfL . — Size rather small. Bill short, stout, obtuse. Tarsus very 
short, stout, arm rough. Tibiae partially feathered. Webs excised. Color 
entirely pure white. 
B. — Xeme^e. 
Size moderate, small, or very small. Body more slender, general organiza- 
tion more delicate. Bill generally slenderer and more acute, the angle not 
very prominent, but acute, the tip decurved and attenuated. Tail variable, — 
even, forked, or cuneate. Legs rather slender. Hind toe always present. 
Head usually with a hood, or with a black ring round the neck. Under parts 
white, with a decided roseate tint. 
5. Chroicocephalus . — Size moderate and very small. Bill slender, the tip 
more or less decurved. Tail even. 
6. Rhodostethia . — Size small. Bill short and very slender. Neck with a 
black ring, but head without a hood. Tail cuneate. 
7. Xema . — Size small. Bill short, rather slender, the angle acute. Head 
with a hood and neck with a ring. Tail moderately forked. 
8. Creagrus. — 11 Of medium size ; bill very strong and much curved ; mantle 
grayish white ; tail deeply forked.” — Lawr. 
The above brief characters define the genera sufficiently for our present 
purposes ; the aim being rather the determination of species than rigid syste- 
matic classification. 
, Genus 1. Larus, Linnseus. 
Larus , Linn. 1744; nec 1735 ; (typus L. canus, fide Gray.) 
Gavia , Moehring, 1752 ; nec auct. 
Leucus , Kaup, 1829. 
Laroides , Brekm, 1830; t. L. argentatus , Briinn. Bp. 1856 ; (typus idem.) 
Plautus, Reichenbach, 1853. 
Glaucus, Bruch, 1853 ; (t. L. glaucus , Briinn.) 
Dominicanus, Bruch, 1853 ; (t. L. marinus , Linn.) 
Gavina , Bp. 1854, fide G. R. Gray; nec Bp. 1856. 
Laroides , Bruch, 1855 ; (t. L. glaucus, Briinn. = Glaucus, Bruch, 1853.) 
Leucus, Bp. 1856 ; (t. L, glaucus , Briinn. Emend. Leuc. Kaup, 1829 = Laroides 
Bruch, 1855, vel Glaucus, Bruch, 1853.) 
The eleven species of the genus found in North America may be very na- 
turally arranged under the following sections or subgenera : 
Section A. — Leucus Bp. ( Plautus , Reich. Glaucus, p. Bruch, 1853, La- 
roides p. Bruch, 1855.) Large and powerful ; primaries without any 
black ; upper parts very light. 
a. Color above entirely white. 
1862.] 
