Tres Marias consist of four small islands lying off the coast of north-western Mexico, about 70 miles from San Blas. The largest is about 15 miles long by 10 wide. They are of horizontally stratified deposits, of moderate height and flat-topped, and everywhere covered with luxuriant virgin forests. They appear to lie within the 100 fathom line of soundings. Fifty-two species of birds, of which 45 were land-birds, were collected on these islands. They consisted of 19 Passeres; 11 Picariæ (7 being humming-birds); 10 Accipitres; 2 parrots, and 3 pigeons. All were Mexican species except 4, which were new, and presumably peculiar to the islands, and one tolerably marked variety. The new species belong to the following genera;--_Parula_ and _Granatellus_ (Mniotiltidæ); _Icterus_ (Icteridæ); and _Amazilia_ (Trochilidæ). A small _Psittacula_ differs somewhat from the same species on the mainland. There are a few mammalia on the islands; a rabbit (_Lepus_) supposed to be new; a very small opossum (_Didelphys_), and a racoon (_Procyon_). There are also several tree-snakes, a _Boa_, and many lizards. The occurrence of so many mammalia and snakes is a proof that these islands have been once joined to the mainland; but the fact that some of the species of both birds and {60}mammals are peculiar, indicates that the separation is not a very recent one. At the same time, as all the species are very closely allied to those of the opposite coasts when not identical, we may be sure that the subsidence which isolated them is not geologically remote. Socorro, the largest of the Revillagigedo Islands, is altogether different from the Tres Marias. It is situated a little further south (19 S. Latitude), and about 300 miles from the coast, in deep water. It is about 2,000 feet high, very rugged and bare, and wholly volcanic. No mammalia were observed, and no reptiles but a small lizard, a new species of a genus (_Uta_) characteristic of the deserts of N.-Western Mexico. The only observed land-shell (_Orthalicus undatus_) also inhabits N.-W. Mexico. Only 14 species of birds were obtained, of which 9 were land-birds; but of these 4 were new species, one a peculiar variety, and another (_Parula insularis_) a species first found in the Tres Marias. With the exception of this bird and a _Buteo_, all the land-birds belonged to different _genera_ from any found on the Tres Marias, though all were Mexican forms. The peculiar species belonged to the genera _Harporhynchus_ (Turdidæ); _Troglodytes_ (Troglodytidæ); _Pipilo_ (Fringillidæ); _Zenaidura_ (Columbidæ); and a variety of _Conurus holochrous_ (Psittacidæ).