[Illustration: FIG. 3. Climatographs for Minatitlan, Veracruz, and Salina Cruz, Oaxaca, based on data given by Contreras (1942). Plotted points are for mean monthly temperatures and rainfall; months are indicated by numbers.] _Vegetation_ The topography and climate combine to produce drastically different types of climax vegetation on the northern and southern lowlands of the isthmus. The picture is somewhat complicated by the savannas on the Gulf Coastal Plain, which, as will be shown later, are dependent upon edaphic features more than climatic conditions. The following brief account of the vegetation in the Isthmus of Tehuantepec is based on data provided by Williams (1939) and Goldman (1951), supplemented by personal observations. The purpose of this description is not to analyze the flora of the isthmus, but to give the reader a picture of this aspect of the biota of the major environments with which I shall be concerned in the ensuing discourse on the amphibians of the region. The three divisions of the isthmus recognized in the account of the physiography serve equally well in describing the vegetation. Those divisions are as follows: Gulf Lowlands On the lowlands north of the continental divide and extending to the Gulf of Mexico are three major types of vegetation--tropical rainforest, arid tropical scrub forest, and savanna. Aside from these, there are marshes and lagoons near the coast. On the coastal dunes there are thickets of sea grape, patches of _Cenchrus_, and clumps or scattered _Opuntia_. The lagoons are bordered by mangrove thickets made up primarily of _Lonchocarpus hondurensis_. In the marshes along the lower Rio Coatzacoalcos and Rio Papaloapan the tall tough grass, _Gynerium sagittatum_, is common. According to Beard (1953: 291) the development of savanna vegetation is dependent upon soil, topography, and drainage. Level regions having permeable soil horizons lying on top of an impermeable horizon provide poor drainage. In most savanna regions in the Americas the grasslands become waterlogged or even partly flooded during the rainy season and desiccated in the dry season. Many ecologists and phytogeographers have postulated that savannas are either man made or are examples of a fire climax. Beard (_op. cit._: 203) provided multitudinous evidence that the association of savanna vegetation and certain types of edaphic and topographic conditions was so strongly marked that grassland is the natural vegetation in these areas. Savannas are scattered through southern Veracruz eastward to British Honduras. These usually are grasslands having scattered trees or clumps of trees around depressions, which may contain water throughout the year (Pl. 1, fig. 1). According to Williams (_op. cit._), the most common trees in the savannas in southern Veracruz are _Ceiba pentandra_, _Chlorophora tinctoria_, and _Byrsonima crassifolia_.