NATURAL SCIENCES OE PHILADELPHIA. 
537 
elongated and more deeply forked, and contained only about twice in the wing. 
Feet short and stout, black; tarsi about two-thirds the bill, or rather less; as 
long as, or slightly longer than, the middle toe and claw. Webs moderately 
incised, the inner the most so. Hind toe very short. 
This genus, as at present constituted, is chiefly distinguished from Sterna by 
its large size and general robust form, stout bill and feet, and (in typical spe- 
cies) much shorter and less forked tail. In the preceding diagnosis I have 
been obliged to define the genus with considerable latitude from the somewhat 
dissimilar types at present retained in it. Thus, if we take the Th. caspius , 
and Th. cantiacus, which may be considered as representing the two extremes of 
form, we shall find great discrepancies in such important features as shape and 
robustness of bill, amount of emargination of tail, &c. ; and regarding these 
extremes alone, might well be inclined to separate them. Examination, how- 
ever, of intermediate species, such as T. regius and elegans , of North America, 
T. velox , of Europe, &c., will show so gradual a transition in nearly every 
feature, from one extreme to the other, that it becomes exceedingly difficult to 
draw a line which shall naturally divide the group into two or more genera. 
In view of the above facts, I prefer, for the present at least, to retain the several 
species under a single genus, as they certainly do differ, markedly, from Sterna 
in important characteristics, although presenting the above discrepancies among 
themselves. 
It is not impossible, however, that T. caspius , with one or two other very 
closely allied species from various parts of the world, may be, without impro- 
priety, separated generically from the others. This species is typical of a 
group, all intimately allied, which are pre-eminently distinguished by their ex- 
ceedingly large, high, robust bills, very stout feet, remarkably short tails, — the 
lateral feathers of which are scarcely at all elongated, and are not tapering nor 
acuminate, — and general large powerful form. The genus might, by the exclu- 
sion of this form, be greatly restricted, and much more rigidly defined. 
Of the five synonyms given at the head of this article, all, with the exception 
of Actochelidon , (the type of which is S. cantiaca :, Gm.) are based upon S. cas- 
pius , Pallas. Of these Thalasseus , of Boie, has priority in point of date, and 
is the name to be adopted for the genus. Boie’s genus, however, is considered 
to be based upon S. caspius , merely from the fact of that species being the first 
mentioned, no particular type being indicated. In the event of the separation 
of S. caspius and its intimate allies, above suggested, it might be well to apply 
the name Hydroprogne to the restricted group, Thalasseus being used to desig- 
nate the remaining species. In view of the very slight reasons for considering 
Thalasseus as having special reference to S. caspius , such a procedure would be 
hardly, if at all, an infringement on the rules of nomenclature, and would ob- 
viate the necessity of presenting regia, elegans and their congeners under a 
generic designation not before employed, — viz. : Actochelidon. 
Thalasseus caspius Boie ex Pall. 
Sterna tschegrava , Lepechin, Nov. Com. Pet. xiv. p. 500. 
Sterna caspia , Pallas, Nov. Com. Pet. xiv. p. 582. Lawrence, Gen. Rep. Birds, 
1838, p. 859, and o’f most authors. 
Thalasseus caspius , Boie, Isis, 1822, p. 563. 
Hydroprogne caspia , Kaup, Sk. Ent. Eur. Thierw., 1829, p. 91. 
Helopus caspius , Wagler, Isis, 1832, p. 1224. 
Sylochelidon caspia , Brehm, Bonaparte, Comp. List, 1838. Lawrence, Proc. N. 
Y. Lyc. Nat. Hist,. 1850, v. 3 T. 
Sterna megarhynclios , Meyer, Taschenb. Deuts., ii. p. 457. 
Diag. — T. rostro maximo, robustissimo, rubro ; palpebris inferioribus albis J 
remigibus griseo-fuscis, supra argentatis, nec intus albis ; cauda breviore, 
emarginata; pedibus validissimis, nigris, digito medio cum ungue tarso 
breviore. 
1862.] 
