506 Mr. N. A. Vicors on the Natural Affinities 
terized by the absence of the hind toe, by means of the species 
Larus tridactylus, Lath., where, though the hind toe is not abso- 
lutely deficient, as might be inferred from the specific name, there 
appears but the rudiment of one, or rather a stump without a nail. 
The last-mentioned genus Halodroma originally belonged to the 
Procellaria, Linn., and was separated from it by its tridactylefoot. 
Even in this character, however, it forms a passage from Larus to 
the groups that compose the genuine Procellaria, all of which are 
distinguished by the singular peculiarity of having no true hind 
toe, but a nail adhering to the tarsus in its place. We thus ar- 
rive at the Petrels, separated into the groups of the Procellaria, 
Auct., Pachyptila, 11l., Puffinus, Ray, and the section denomi- 
nated by M. Temminck ** Les Petrels Hirondelles.” These two 
latter groups appear to lead us back to the Terns, or Sea Swal- 
lows, from whence we started. The whole of this family, which 
corresponds with the ** Longipennes” of M. Cuvier, is distinctly 
characterized by the strength and expansiveness of their wings, 
with the aid of which they traverse immeasurable tracts of the 
ocean in search of their food, and support their flight at consi- 
derable distances from land, seldom having recourse to their 
powers of swimming. We may thus discern the gradual suc- 
cession by which the characters peculiar to the order descend 
from the typical groups that swim and dive well and frequently, 
but make little use of their wings for flight, to the present groups, 
which are accustomed to fly much, but seldom employ their 
powers of swimming, and never dive*. The family of Laridemay 
thus be observed to stand at the very extremity of the order, and 
it assumes, as I have already observed, in conjunction with the 
* * Le plus grand nombre des espèces qui composent les premiers genres de cet 
ordre (sc. Sterna, Larus, Procellaria), se reposent sur la surface de la mer, volent le 
plus souvent, ne nagent point habituellement, et ne plongent jamais." Temm. Man. 
p. 730. : 
other 
