vely
low connection with the Cordillera Volcanica, the Sierra de Coalcoman is
isolated from other mountain ranges in southwestern Mexico.

CLIMATE

The climates in Michoacan vary from tropical in the lowlands to cool
temperate at high elevations in the Sierra de Coalcoman and Cordillera
Volcanica. The highest temperatures are known in the Balsas-Tepalcatepec
Basin, where at Churumuco the mean annual temperature is 29.3 deg. C. and
the range of monthly means is 3.5 deg. C. (Contreras, 1942). Frosts occur
sporadically on the Mexican Plateau, and in the winter snow falls on the
highest mountains.

Precipitation varies geographically and seasonally. Most of the rain
falls between June and October. In the Balsas-Tepalcatepec Basin
rainfall in the rest of the year is negligible. The annual average
rainfall at Coahuayana on the Pacific Coastal Plain is 871 mm.
(Guzman-Rivas, 1957:52). In the Balsas-Tepalcatepec Basin rainfall
seldom exceeds 800 mm. per year. In the mountains precipitation is
heavier and somewhat more evenly distributed throughout the year, but
still definitely cyclic. For example, Uruapan (elevation, 1500 meters)
receives an average annual rainfall of 1674 mm. (Contreras, 1942). The
prevailing winds are from the Pacific Ocean. The southern (windward)
slopes of the Sierra de Coalcoman probably receive more rain than any
other part of the state. The Balsas-Tepalcatepec Basin lies in a rain
shadow of the Sierra de Coalcoman, and the Mexican Plateau lies in a
somewhat less drastic rain shadow of the Cordillera Volcanica; these are
the driest regions in the state.

VEGETATION AND ANIMAL HABITATS

For the purposes of this report I have adopted the classification of
types of vegetation that seem to me most significant in terms of
ecological distribution of reptiles and amphibians in Michoacan. These
types are as follows:

TEMPERATE (1000-4000 meters)
Fir Forest (2400-4000 meters)
Pine-oak Forest (1000-4000 meters)
Mesquite-grassland (1500-2100 meters)

TROPICAL (0-1000 meters)
Arid Tropical Scrub Forest (0-1000 meters)
Tropical Semi-deciduous Forest (150-600 meters)

The vegetation of the Pacific Coastal Plain and the Balsas-Tepalcatepec
Basin consists of arid tropical scrub forest, composed of deciduous
trees, which in many places are stunted and widely spaced. In the dry
season there is little cover provided by this forest. In the rainy
season there is a sparse