506 Mr. N. A. Vigors 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, 111,, 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 Laridcemay 

 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 especes 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. 



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