The genus Citromyces Wehmer comprises a number of species which were considered by Wehmer to be unique in the fact that they produce considerable quantities of citric acid by the fermentation of sugar. Because of this, Wehmer gave to the genus the name of Citromyces . The genus is closely related to Penicillium and Aspergillus , and has in fact been dropped by Thom in his recent book on the genus Penicillium (1929).Thom prefers to regard all species previously regarded as Citromyces , as species of Penicillium (see Part IY). Physiologically the genus Citromyces was considered to be distinguished from other genera by the production of citric acid, but more recent work has shown that not only is the production of citric acid a fairly common characteristic of other genera, particularly of Aspergillus and Penicillium , but that, quantitatively, certain species which are not Citromyces give much larger yields of citric acid.