NATURAL SCIENCES OF PHILADELPHIA. 541 



not disappear at this season, remains of an unmixed brownish black. The 

 lateral tail feathers are shorter. The bird otherwise as in summer. 



At all seasons the yellow tip of the bill varies in extent, and it also presents 

 a varying regularity and sharpness of division from the black. I am inclined 

 to think that the extent of the yellow depends upon the age of the bird: its 

 intensity upon the season. The longest yellow tip before me measures three- 

 fourths of an inch, the shortest one-fourth. In a large series of specimens the 

 tarsi and toes scarcely differ appreciably. The markings of the primaries, in 

 their extent and disposition, are also remarkably constant. The variation in 

 length of wing from flexure in adult summer birds is about half an inch. The 

 tail varies somewhat in depth of fork, but is always less than in the species of 

 Sterna proper. 



A series of winter skins from Jamaica in, probably, their first moult, differ 

 from adult examples from various points on the Atlantic Coast in being every 

 way considerably smaller. The bills are about a third of an inch shorter than 

 the average ; and other parts differ proportionally. 



The American Sandwich Tern was first separated from the European by 

 Cabot, (1. c.) in 1847. Most of the points of difference, however, assigned by 

 that writer, disappear when large series from both continents are compared. 

 The difference in the measurements given exists equally in individuals of both 

 species ; for, as will be seen from the above remarks, specimens vary greatly in 

 these respects. After an attentive examination of a large number of skins, I 

 can appreciate no differences whatever in these respects ; and in size and pro- 

 portions, of bill as well as of the whole body, the two appear identical. Neither 

 can distinctive characters be drawn from the yellow tip of the bill. In both 

 species the line of union of the yellow and black is equally irregular, depend- 

 ing for its exact character on the age of the bird. In both, the yellow runs 

 along the gonys, nearly or quite to the angle at the symphysis. It also extends, 

 but in a less degree, along the ridge of the upper mandible, and even for a little 

 way on the cutting edges of both mandibles. The outline of the yellow on 

 the sides of the bill is also more usually concavo-convex than perfectly straight 

 and perpendicular. The trenchant line of union, which existed in the speci- 

 men described by Cabot, must have been rather exceptional. I cannot appre- 

 ciate any difference in the width of the bills of the two in the series before me. 

 A discrepancy in the claws of the two does not exist as constant. 



We are reduced, therefore, in separating the two birds, to the single remaining 

 character given by Cabot, that of the primaries. These parts in the 

 American bird are not darker than those of the European, since their color de- 

 pends on their age ; but a decided difference in the white margins of the inner 

 webs exists uniformly in ail the specimens from either country that I have ever 

 examined. In the European bird the white of the inner web of the first pri- 

 mary occupies at the base nearly the whole of the web, the dark portion being 

 merely a narrow line along the shaft. This black portion widens but little as 

 it runs along the feather, so that the white border extends quite broadly to the 

 very tip of the feather, which it entirely occupies. In the American, on the 

 contrary, the black portion is in its whole length wider, and, about one and a 

 half inches from the tip becomes quite suddenly very decidedly broader, so 

 much so as nearly to cut off the white, which latter continues forward a little 

 further, but only as a very narrow bordering line, and finally disappears before 

 it reaches the tip. The same holds good, though somewhat less markedly, of 

 the second, third and fourth primaries. The following would therefore con- 

 stitute the 



Differential diagnoses of the American and European Bird, 



Th. cantiacus. White margins of inner web of outer three or four prima- 

 ries wide, extending quite to tip, which it wholly occupies. Breadth of white 

 portion l inches from tip of first primary, -25 of inch. 



1862.] 



