LARID.E — THE GULLS AND TERNS — PAGOPHILA. 197 



the diiterence in I'orm is not less striking than that of size. The genus Sterna offers scarcely less 

 of a contrast between the large, Gull-like S. cassia and the minute ii. antillarum. In order to 

 separate the more marked variations of form in either of these genera, however, it would be neces- 

 sary to name a larger number of subdivisions than most authors would recognize as distinct genera. 

 Notwithstanding this fact, we are convinced that, while such a procedure undoubtedly simplifies 

 the nomenclature, it by no means expresses the true relationship of the forms so designated to call 

 all the square-tailed Gulls (excepting Pacjophila and Eissa) Larus, and all the fork-tailed Terns 

 with full}' webbed-feet Sterna. In fact it is only from want of suitable material that we have not 

 attempted a subdivision of the genera Larus and Sterna in their comprehensive sense. Allowing, 

 therefore, each the fullest possible scope, we submit the following analysis of the North American 

 genera of Laridas : — 



Lari7ice. 



1. Pagophila. Tail even ; hind toe perfectly developed, though small ; tarsus shorter than 



the middle toe and claw, serrate behind. Color entirely white, the young sparsely spotted 

 with dusky. Size medium. 



2. Rissa. Tail even, or slightly emarginate ; hind toe rudimentary, or altogether absent ; 



tarsus much shorter than the middle toe without its claw, not serrate behind. Above, 

 pearl-blue, beneath white ; young similar, but with a black nuchal jjatch (and in one 

 species a black shoulder-patch). Size medium. 



3. Larus. Tail even ; hind toe always well developed ; tarsus always longer than the middle 



toe with its claw, not serrate behind. Size and coloration extremely variable, but young 

 always A'ery different from the adults. 



4. Rhodostethia. Tail graduated, or wedge-shaped. Size small. Adult pearl-blue above, 



rosy white beneath and on head and neck, the latter encircled by a black collar. 



5. Xema. Tail forked. Size small. Adult pearl-gray above, white beneath, including the 



neck all round, the head dusky. 



Sternince. 



6. Sterna. Tail decidedly forked ; webs of the toes filling the greater part of the interdigital 



spaces, but both with a concave or scalloped anterior outline. Size extremely variable. 



7. Hydrochelidoii. Tail emarginate ; webs of the toes very deeply scalloped, occupying 



much less than half the interdigital space. Size small. 



8. Anous. Tail graduated, or wedge-shaped ; webs of the toes completely filling the inter- 



digital spaces, and scarcely or not at all scalloped in front. 



Genus PAGOPHILA, Kaup. 



Gavia, Boie, Isis, 1822, 563 (type, Larus eburneus, Phipps).^ 



Pa(jo2}hila, Kaup, Nat. Syst. Eur. Thierw. 1829, 69 (type, Larus eburneus, Phipps). 



Cctosparactes, Macgill. Man. Brit. Oru. II. 1842, 251. 



Char. Size medium ; tail even ; hind toe well developed, though small, the nail relatively 

 large ; tarsus shorter than the middle toe and claw, roughly granular or almost serrate behind ; 

 color entirely white in the summer adult ; white, sparsely spotted with dusky, in the winter 

 plumage (and young?). 



The genus Pagophila contains but one well established species, although several nominal ones 

 have been recognized, all of which were probably based upon special stages, or somewhat abnormal 

 individuals, of P. eburnea. 



1 It is quite probable that a proper adherence to the rules of nomenclature will require the use of Gavia 

 for this genus instead of PafjfqMla ; but at present we are unwilling to make the change. (Cf. Stejxeger, 

 «« Proc. U. S. Nat. Mus." Vol. 5, p. 39.) 



