The third known type of archaebacterium is the thermoacidophile, and the members of this group too are nota ble for their habitat. Sulfolobus, one of the two genera of thermoacidophiles, is found in hot sulfur springs. Its various species generally grow at temperatures near 80 degrees Celsius (176 degrees Fahrenheit); growth at temperatures above 90 degrees has been observed for some varieties. Moreover, the springs in which Sulfolobus flourishes are extremely acidic; pH values lower than 2 are common (a pH of 7 is neutral). Thermoplasma, the other genus of thermoacidophile, has so far been found only in smoldering piles of coal tailings. It is a mycoplasma: it has no cell wall but merely the limiting cell membrane.