In regard to the _fauna_, there are in Costa Rica about one hundred and twenty-one species of mammalia, of which ten are domesticated and four of _Mus_ introduced, leaving 107 as indigenous to Costa Rica. There are only a few species peculiar to Costa Rica, and also but a small number peculiar to Central America, among which are the _Tapirus dowi alston_ and three species of monkeys. About one-fifth of the total number also belong to South America and one-seventh to North America. The rest are found as well in North as in South America. With respect to the avifauna, there are 725 known species. This great variety of the avifauna is due to especial climatic conditions, to the very rich flora, to the geographical position between two oceans and to the vicinity of so many islands of the Caribbean Sea. It is composed of 67 Neoarctic species, which are also found in the north of Mexico; of 247 Neotropical or South American species, of 260 autochthonous or exclusively Central American species, and 128 newly described species which live as well in the northern as in the southern continent. The rest, comprising 23 species, have a doubtful origin. The best singing birds are the Gilguero, Yigüerro, Toledo, Mozotillo, Cacique, Mongita, Comemaiz, Setillero and Agüillo. There are over 130 species of _Reptilia_ and _Batrachia_ in Costa Rica. Those known and described are 36 _Batrachia_, 28 _Lacertilia_, 60 _Ophidia_ and 6 _Testudinata_. Poisonous snakes are the Toboba, Bocaracá, Oropel, Terciopelo and Cascabel. Costa Rica is also very rich in _Fishes_. Those in the Pacific are almost entirely different from those of the Atlantic Ocean. Also its tributary waters have more varied species than those of the Atlantic slope. In correspondence with the varied topographical, climatological, and botanical conditions of Costa Rica is also the _invertebrate fauna_. And here the National Museum, under Mr. Anastasio Alfaro, and the “Instituto fisico geografico Nacional.” under Professor H. Pittier, are doing equally excellent work in bringing them to our knowledge, as they have done like service in other branches of Natural History.