Lying in a rain shadow cast by the Tuxtlas and on sandy and well-drained soils is a dense xerophytic forest. The crown of this deciduous forest usually is little more than ten to twelve meters above the ground (Pl. 1, fig. 2). Conspicuous trees in this scrub forest are _Acacia cornigera_, _Bauhinia latifolia_, _Calliandra bijuga_, _Cassia laevigata_, _Guazuma ulmifolia_, and various species of _Bursera_. The most extensive type of vegetation on the Gulf Coastal Plain is a tall evergreen forest resembling tropical rainforest. Although this forest is made up of many species of trees that are characteristic of true rainforest, the forest on the Gulf Coastal Plain cannot be classified as true rainforest, neither by the climatic conditions, nor the structure of the forest. The seasonal variation in rainfall probably is the chief factor in hindering the development of a rainforest climax vegetation. Usually a minimum of 65 mm. of rainfall each month is considered essential for the development of true rainforest. At Minatitlan the average rainfall for March (39 mm.) and April (36 mm.) is far below this minimum. Structurally, this forest has a crown about 30-35 meters above the ground but individual trees rising five meters or more above the crown (Pl. 2, figs. 1-2). There is no clear stratification within the forest; in many parts of it there are dense growths of bushes, small trees, and palms. The forest on the Gulf Coastal Plain, therefore, most properly might be referred to as a quasi-rainforest, a term that has been applied to other such forests in tropical America. Among the abundant and dominant trees in this forest are _Swietenia macrophylla_, _Calophyllum brasiliense_, _Achras zapota_, _Ceiba pentandra_, _Castilla elastica_, _Cedrela mexicana_, _Tabebuia Donnell-Smithi_, _Calocarpum mammosum_, _Bombax ellipticum_, and a variety of _Ficus_. Epiphytes and Ilianas are abundant. Central Ridges The vegetation of the central ridges of the isthmus is, for the most part, transitional between the tall rainforest of the Gulf Coastal Plain and the low xerophytic scrub forest of the semi-arid Pacific Coastal Plain. On the northern slopes of the ridges the rainforest is more poorly developed than on the plains to the north. Many of the same species of trees are present, including _Ceiba pentandra_, _Cedrela mexicana_, _Swietenia macrophylla_, and _Ficus_ sp.; nevertheless, these seldom are as large as members of the same species in the forest on the plains. Other species present on the forested slopes include _Tabebuia Donnell-Smithi_, _Zanthoxylum melanostictum_, _Pithecolobium arboreum_, and a species of _Pterocarpus_. The structure of this forest differs from that on the Gulf Coastal Plain in that there is no continuous upper canopy and there is a dense undergrowth (Pl. 3, fig. 1). This type of forest extends from Mogone southward to about Matias Romero.