SINCE the discovery of the Mount Lyell salamander, Hydromantes platycephalu.s (Camp), in 1915 (Camp, 1916), information on the natural history of this relict species has remained meager. The greatest contribution has been Adams' paper (1942) clarifying its systematic position and presenting personal observations, together with a summary of the scanty literature on the distribution, habitat, and habits of the species. Recently, I have had opportunity to study the behavior of a dozen Mount Lyell salamanders. My observations have served to reinforce earlier impressions regarding the use of the feet and tail in locomotion. The tail action appears to be unique among North American salamanders. To appreciate the adaptive significance of the structure and action of the tail and feet, one must consider the nature of the habitat occupied by these salamanders. The species is known only from the Sierra Nevada of California to which it is probably restricted. All collection sites (7) are in the Boreal Life-Zone except for two localities in the upper Transition Zone in Yosemite, which may represent accidental distribution to lower altitudes as a result of individuals falling from precipitous places (Adams, 1942: 192). Soil conditions, according to Adams, are remarkably similar at the collection sites, consisting of decomposed granite with little humus. Regions occupied by the species are characterized by granite rock exposures, often including both weather-roughened and glacial-polished surfaces. Talus is abundant, and-rock fissures are common. Successful locomotion in the rugged habitat requires considerable climbing ability. That climbing ability is important for the survival of the species in at least some parts of its range is suggested by the situation under which the animals occur on the top of Half Dome at the east end of Yosemite Valley, Mariposa County, California. The existence of the animals on this Boreal island1 is made possible by the presence of a deep vertical fissure (and perhaps other fissures) at the top of the Dome. This crack is about 11/2 feet wide at the surface. Its exact depth is unknown, but it is probably several hundred feet. With the melting of the snow that caps the peak (largely gone by early July), the salamanders must retreat to considerable depths to escape desiccation and overheating. The loose exfoliating shells of granite that litter the surface, although resorted to as cover when snow patches are still present, do not provide sufficient protection against drying during the intense illumination and aridity of summer. The area is almost completely barren of vegetation that might provide shade. Presumably many of the animals enter the fissure to seek levels where moisture and temperature conditions approach optimum. Water that enters the crevice from melting snow and occasional rains provides the necessary moisture. It would seem, then, that the survival of the animals on Half Dome depends, among other things, on their ability to ascend and descend the rough and smooth, variously inclined but largely vertical, surfaces of this crevice.