A large percentage of the species of Myxophyceae collected by Dr. Wille on Porto Rico are unicellular. Many of these species belong and may readily be associated with more or less well-recognized and clearly defined genera. Others seem decidedly to be allied with less clearly delimited genera and before these can be satisfactorily placed the limits of such genera must be either more clearly defined and amplified or new genera must be erected for their reception. In this instance I shall attempt the former procedure. Three of such genera are Chroococcus Naegeli, Gloeocapsa Kuetzing, and Anacystis Meneghini.
Naegeli proposed the genus Chroococcus in 1849,2 selecting as the type of the genus C. rufescens (Pleurococcus rufescens Breb.) at the same time recognizing nine other previously described species, which had been placed with one or the other of the genera Protococcus or Pleurococcus. All of these have since been very generally recognized as belonging to Chroococcus. Naegeli's conception of the genus as set forth in his diagnosis and in his discussion which follows seems to have been of a group whose cells exist either singly or, by division "in alien Eichtungen des Eaumes" and remaining for a time intact, become associated into small, spherical or cuboidal families, or colonies of 2—8 or, more rarely, 16-32 cells, surrounded by a thin colorless, homogeneous or at times slightly lamellose wall. Although Naegeli states that the wall is thin, scarcely one-third the thickness of the lumen of the cell (a thick wall as compared to the cell proper) he undoubtedly meant thin as compared to that of Gloeocapsa, which he discusses in connection with Chroococcus, and which may become several times thicker than the lumen of the cell.
The genus Gloeocapsa was proposed by Kuetzing in 1843. He designated no type species, but the first species mentioned is G. montana, which may ordinarily be considered the type of a genus when no special one is proposed. His diagnosis of the genus is very brief and incomplete. In 1846,4 he illustrated the species.
The genus was reviewed by Naegeli (loc. cit.), who designated as the type G. atrata Kuetz., which he redescribed and illustrated. I have examined bits of the type of both of these species and judging from these and from the descriptions and figures given by Kuetzing and Naegeli, both writers had practically the same conception so far as the type, G. atrata, is concerned. As pointed out by Naegeli and by subsequent writers, the two genera Chroococcus and Gloeocapsa undoubtedly overlap, and such forms as G. montana seem as clearly allied to the one as to the other. I am considering forms with a firm, smooth, hyaline wall, or tegument, Avith relatively few cells in a colony, and these remaining angular after division throughout the greater part of the life cycle of the colony, and such forms as remain within special teguments for several generations all remaining within a common, more or less gelatinous, original.