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Show 1906.] IN MEXICAN LIZARDS. 279 as to the physical features. I have to thank the authorities of that splendid museum for their liberality in sending over to Cambridge the greater number of their Cnemidophori for minute examination. These were supplemented by the study of the specimens in the British Museum, where, as usual, I had the inestimable benefit of m y friend Boulenger's critical advice and never-failing help. Some Berlin types have also been examined. The total of Cnemidophori studied for the purpose of this paper amounts to some 520 specimens, from the United States to the Isthmus of Tehuantepec; about 450 of these are detailed in the appended tables. Adding about 40 from South America in the British Museum, studied cursorily for general comparison, the whole amounts to some 560 specimens, apparently sufficient for all purposes, but in reality not so, since, for instance, the whole tessellatus-grou]) is but meagrely represented. The whole range, from the Isthmus to Utah, is enormous, more than 2000 miles; and even if we restrict ourselves to Mexico, the 500 specimens are crowded into comparatively few districts and leave many large regions blank. Such a blank is, for instance, the country from Colima to Acapulco, 300 miles. For the whole of Mexico proper, excluding Yucatan and Lower California, scarcely 60 localities are on safe record. A single locality, Hermosillo, represents the whole large State of Sonora, and Presidio near Mazatlan the State of Sinaloa. Mexico is an ideal country for the study of geographical distribution, because it contains, often in juxtaposition, vast semi-deserts, high plateaus, big continuous ranges of mountains with peaks in the eternal snow, hot lowlands of the Atlantic or humid type with luxurious rain forests, and of the Pacific or drier type; large forests of pines, oaks, or of tropical trees ; rivers and lakes ; regions of enormous fertility and hopeless deserts. In short, every climate and every conceivable kind of bionomic conditions are represented in this country. No wonder that this diversity is expressed in the well-nigh endless, kaleidoscopic variations of the genus Cnemidophorus, the main genus of strictly humi-vagous Lizards of the country. This Tejid genus is invaluable for the study of variation. It is so plastic within its well-defined generic characters, that it is represented by some form or other in almost every kind of terrain. Its highest altitude above sea-level seems to be reached near 7000 feet, as shown by its occurrence near Santa Fe in N e w Mexico. In Mexico its highest record is 7100 feet near Puebla; it is absent in the Valley of Mexico, about 7400, and at Amecameca 8000 feet, but it reappears at San Juan del Rio 6300, Celaya 5800, Acam-baro 6000, Patzcuaro 6700, Durango 6200, Chihuahua 4700 feet. These localities show that the lizards are not averse to moderate altitudes, but all these places are situated on some kind of plateau. On more isolated mountains the lizards seem to stop at a lower level. For instance, on the eastern slopes of the Nevado de Colima they stop at 5100, on the Cerro de San Felipe near |