[2] We have not included within the scope of the present work the territory of Santa Cruz and Tierra del Fuego, which are politically part of the Argentine Republic, but only Northern Patagonia down to the Rio Negro. The subjoined Table contains in the first column the number of Neotropical species assigned to each of the 20 different Orders of Birds in the 'Nomenclator Avium Neotropicalium.' In the second column is given the corresponding number of Argentine species of each order according to the present work. The total number of Neotropical species will be seen to be about eight times as many as those of the Argentine Avifauna; so that where this average is exceeded in the individual orders the particular group may be considered to be over-represented, and where it is not reached to be under-represented in the Argentine Avifauna. This is of course a very rough-and-ready mode of calculation, because it assumes that every species has an equal area of distribution, which is not ordinarily the case; but it will serve to give us some general ideas on the subject. We will now proceed to consider the principal groups one after another, and to point out their comparative importance in the Argentine Avifauna. The known Argentine Passeres, according to the present work, are 229 in number; those of the Neotropical Region, according to the 'Nomenclator,' are 1976; so that the species of Argentine Passeres, according to the ratio which we have adopted, would be very little short of the average numbers. If, however, we divide the Passeres up into the three suborders of Oscines, Oligomyodæ, and Tracheophonæ, we shall find that the Oligomyodæ are rather in excess of the estimated average, while the species of the two other groups are deficient. This arises mainly from the large number of Tyrannidæ belonging to the Argentine Ornis. Not less than 63 species of this group have been already discovered within its limits. Besides Tyrants, other prevailing families of Passeres in the Argentine Avifauna are the Finches (Fringillidæ) and the Wood-hewers (Dendrocolaptidæ), both of which have 46 representatives in the Argentine Republic. Both these families are, however, well represented all over the Neotropical Region. But the Tapacolas (Pteroptochidæ) and the Plant-cutters (Phytotomidæ) are both of them families specially characteristic of the Patagonian Avifauna, and both of them are represented in the Argentine Republic. One other Passerine form is also worthy of special mention. The Dippers (Cinclidæ) are essentially an Arctic group diffused all over the Palæarctic and the Arctic Regions. It is singular, however, that this form should reoccur at higher elevations in the Neotropical Region. A _Cinclus_ is found in the mountains of Colombia, another in the Andes of Peru, and a third has lately been discovered in the northern sierras of the Argentine Republic. As regards Passerine birds, therefore, which great order makes up more than one