8 PROF. W. LILLJEBORG ON THE CLASS OF BIRDS. [Jan. 9, 



the branches of trees*. The hinder extremities or the legs exhibit, 

 in conformity with this, three different forms. This induces us to 

 divide the class of birds into three primary groups or subclasses : — 

 1, Natatores; 2, Cursores ; 3, Insessores. Those belonging to the 

 third group generally move more with the assistance of their wings 

 than the others, except some forms of the Natatores, and show gene- 

 rally a higher development of the bird-type. This group also fur- 

 nishes the greatest variety of forms. The Natatores include about 

 550 species, the Cursores 900, and the Insessores 6900 (Bonaparte). 



Nitzsch has, in the treatise referred to, divided the class of birds into 

 three groups : Aves aerese, Aves terrestres, and Aves aquaticse, which 

 in a reverse order correspond with the three groups here arranged ; 

 but he differs from us in including the Columbine birds among the 

 Terrestres, and the Grallatorial birds among the Aquaticse, and in 

 considering the Struthionine birds a distinct group from the other 

 three. 



There is, as far as experience yet extends, a very remarkable cor- 

 respondence between the nature of the upper wing-coverts and of 

 the carotides communes, which adds to the importance of both these 

 characters, which have generally been but little observed. All those 

 birds that have the large upper wing-coverts of the first row on the 

 cubitus so short that they do not reach beyond the middle of the 

 cubital quills, have only one carotis communis, viz. the sinistra. 

 Those birds in which the above-mentioned wing-coverts form several 

 rows and extend beyond the middle of the cubital quills, have, on the 

 contrary, generally two carotides communes, viz. one dextra and one 

 sinistra. The only exceptions to this rule are Cypselus, Trochilus, 

 Merops, one or a few species of Psittacus, Rhea, Phoznicopterus, 

 Podiceps, and Pelecanus, which, although belonging to the latter 

 category in regard to the wing-coverts, yet have only one carotis 

 communis. This is the dextra in Phcenicopterus. We do not, there- 

 fore, hesitate to consider these two characters to be among the most 

 important in judging of the affinity of the birds ; and they show with 

 certainty that the birds of prey have not their place at the beginning 

 or at the end of the system. 



The Strisores, one of the twelve orders in which we have arranged 

 the class of birds, includes several birds that we formerly considered 

 should belong to the Passeres, from their near correspondence in form 

 with the latter. But as they deviate from them in regard to the 

 upper wing-coverts and the claw of the hind toe, and sometimes 

 even in regard to the carotides communes, we are of opinion that they 

 should be regarded as belonging to a different order. They have 

 been separated from the Passeres by Sundevallf and by Nitzsch ; 

 and the former has arranged them under the order Coccyges, which, 

 according to him, also includes the Zygodactyli and Columbce. 

 They are, however, distinct from the Zygodactyli in the nature of 

 their feet, and cannot be arranged under this order without depriving 



* The second mode appears an intermediate link between the first and third. 

 t Kongl. Vetensk. Acad. Handl. 1843, pp. 375 & 376. 



