It is striking that apart from coumarin, hydrocoumarin, and dicoumarol, all naturally occurring coumarins have an oxygen atom at the 7-position i. e. they can be regarded as derivatives of umbelliferone, which is one of the most widely distributed compounds of this class. The simpler members of the group are hydroxylated or alkoxylated coumarins derived from polyhydric phenols containing a resorcinol system,—a fact which accounts for the prominence of the umbelliferone nucleus; thus aesculetin is derived from hydroxyquinol, daphnetin from pyrogallol, citropten from phloroglucinol, and fraxetin from 1:2:3:4-tetrahydroxybenzene. Hydroxyl groups have not been found in position 3 of the coumarin system, and only two authentic cases (dicoumarol and ammoresinol) are known with hydroxyl in position 4. Frequently the oxygen at site 7 is combined in a third ring which may be fused in either of the two possible positions with respect to the α-pyrone system. Only one case, that of alloxanthoxyletin, is known where any other oxygen atom is concerned in the formation of the third ring (pp. 281).