During the past ten years we have examined large numbers of specimens of this group in the field and in American and European herbaria. Most of the more than one thousand type specimens involved in the nomenclature were found and studied. Many of these proved to be representatives of the twenty-eight species listed here; the remainder are material of Chlorophyceae, Rhodophyceae, bacteria, fungi, and other plants and animals. The following synopsis and keys are offered as a summary of the chief morphological features of the various taxa. It is hoped that a detailed revision, complete with synonymy and lists of specimens, can be published in a short time.