In the collections of the United States National Museum are several species of amphibians sent to the museum from Tehuantepec by Francis Sumichrast. These include _Bolitoglossa platydactyla_ (USNM 30305, 30344-6, 30528), _Bolitoglossa rufescens_ (10042), _Chiropterotriton chiropterus_ (30347), _Lineatriton lineola_ (30353), _Parvimolge townsendi_ (30352), _Pseudoeurycea cephalica_ (30350), _Thorius pennatulus_ (30348-9), _Hyla miotympanum_ (30302-3), and _Hyla picta_ (30304). Because of the poor condition of the specimens, determinations of those listed as _Bolitoglossa rufescens_ and _Pseudoeurycea cephalica_ are uncertain. With the exception of the _Bolitoglossa rufescens_, which is stated to have come from Santa Efigenia, all of these specimens are catalogued as having come from Tehuantepec. None of these species has since been recorded from the Pacific slopes of the isthmus; however, they all occur in the vicinity of Orizaba, Veracruz. Probably Sumichrast carried the specimens with him from Orizaba, his home before moving to Santa Efigenia, and shipped them from Tehuantepec to the United States National Museum. These species definitely should not be considered as inhabitants of the Pacific slopes of the Isthmus of Tehuantepec. DESCRIPTION OF THE ISTHMUS OF TEHUANTEPEC The Isthmus of Tehuantepec is a strip of land forming a low pass, which separates the mountain masses of Mexico proper from those of Central America, and at the same time provides a continuum of lowlands from the Gulf of Mexico to the Pacific Ocean. This topography combines with the climatic conditions to create extremely diverse environments, the distribution of which can be adequately understood only after an acquaintance with the topography and climate of the region. _Physiography_ In east-central Oaxaca the mountain masses comprising the Sierra Madre Oriental and the Sierra del Sur terminate in a series of ranges--Sierra de Juarez, Sierra de los Mijes, and Sierra de Choapam. From lofty peaks, such as Cerro de Zempoaltepetl (3400 meters), the highlands diminish eastward to succeedingly lower ridges, until in the middle of the Isthmus of Tehuantepec the continental divide is about 250 meters above sea level. Eastward from this low divide the land rises to form the Sierra Madre de Chiapas, which is continuous with the highland masses of Guatemala. For the purposes of this description, the lowlands of the isthmus may be divided into three parts--the Gulf Coastal Plain, the central ridges, and the Pacific Coastal Plain, which in the isthmus is called the Plains of Tehuantepec (Figs. 1 and 2).