of cytoplasm enclosing a large vacuole. The cytoplasm contained a nucleus and a plain or lobed chromatophore. The appearance of separate oval or round chloroplasts (Fig. 13) was sometimes given in surface view, or an almost complete green mantle lined the cell.

Changes Accompanying the Active Growth of the Coenocyte. These were most marked at the beginning. The oval cells rapidly elongated and the chromatophore, failing to keep pace, often appeared as a girdle (sometimes broken) round the central region, leaving a hyaline portion at each end of the cell. At this stage the chromatophore may still be simple or lobed (Figs. 15 and 16) but if conditions are favourable it soon extends to the ends of the cell. The number of nuclei increases and more pyrenoids are formed (Fig. 17). The chromatophore here is like a mantle with various-sized, rounded lacunæ, but sometimes it takes the form of a large-mesh network. At this stage, when the cell is cylindrical in form, it closely resembles the coenocyte of H. reticulatum in structure. Yamanouchi1 says the cell of H. africanum does not grow into an ellipsoid cylinder but remains spherical or oblong. In the laboratory cultures here, the coenocytes always passed through this cylindrical stage.

Fig. 18 shows a coenobium about 6mm. in diameter which has cylindrical coenocytes, and still larger coenobia with the same form of coenocyte were seen. It is, however, only a temporary form and sooner or later the median region of the cell begins to expand more than the ends and an oval or spherical coenocyte is produced (Fig. 19), the structure of which has already been described.

The number of coenocytes forming the coenobium varied, as many as two hundred were counted in the cultivated material. This is a larger number than that obtained by Yamanouchi who found about sixty in each colony forming an irregular net, but not so large a number as that occurring in some of the coenobia in their native habitat.

I. Yamanouchi, Sh. loc cit. p. 74. 2. Yamanouchi, Sh. loc cit. p. 79.

The form of the coenobium obtained in the laboratory cultures was the normal disc- or saucer-shape. Irregularities occurred but the main plan was quite clear; a circular network, in one layer, was formed and in this, the inner cells were in contact with two or three others at each end, forming pentagonal or hexagonal meshes, while the boundary cells formed a continuous chain.

When the cells became spherical they sometimes remained in contact for a considerable time, and increased in size. The size varied in different coenobia and even in the same coenobium. Sooner or later they became separated and continued to exist in an arrested condition or further growth took place after an arrested period.

In conclusion I wish to acknowledge my indebtedness to Professor W. H. Lang for allowing me to investigate some dried mud from the vleis, sent to him by Miss Stephens of Cape Town University, and for facilities to work in the Cryptogamic Laboratory of the Manchester University.

I should also like to thank Miss Stephens for information about the natural occurrence of H. africanum and for her readiness in supplying material.

SUMMARY.

1. A sexual reproduction by the apposition of swarm-spores to form a coenobium inside the mother-coenocyte was not observed in H. africanum, but evidence of gamete-formation was obtained.

A dissimilarity in the size of the gametes was noted. This requires further investigation under more natural conditions.

2. Resting-spores (hypnozygotes or parthenospores) which varied in size, but were mostly larger than those described in H. reticulatum, were found in dishes where the coenocytes had been allowed to dry and remain undisturbed for some months.

3. Under favourable conditions the production of coenobia from the resting-spores took place much more rapidly than is the case in H. reticulatum. Coenobia began to appear in less than a week after the submergence of the spores in water.

4. The resting-spores gave rise directly to coenobia without

the intervention of the large-zoospore stage described by Pringsheim for H. reticulatum.

5. The normal saucer-shaped coenobia were produced. The individual cells, after passing through a cylindrical stage, became spherical and dissociating, existed as isolated coenocytes.

6. The developmental stages of the coenocyte indicated the formation of a reticulate chromatophore comparable with that found in H. reticulatum.

XII The Harknessellinæ.

By B. B. BANCROFT, B.A., M.Sc.
(Communicated by Mr. S. H. Straw.)

PART I.

Descriptions of Genera and Species and General Description of Ribbing.

INTRODUCTION.

THE first part of this paper is devoted to the description of new genera and species embraced in Harknessella, Reed,1 s. 1., and the general description of the rib-system in that group. The new species which are generally, but incorrectly, referred to Harknessella vespertilio (J. de C. Sow)2 were selected from a considerable number of undescribed forms in my collection. All are from the Bala Series of East Shropshire, and may be regarded as representative types of the new genera.

The second part of this paper will deal with the application of the notation3 to all known forms of ribbing occurring in these species, with the statistical method of description and with considerations of a theoretical nature relating to the rib-system.

THE SUB-FAMILY HARKNESSELLINÆ.

The Harknesselline is proposed as a sub-family of the Rhipidomellida to embrace the assemblage of species hitherto referred to Harknessella, Reed. If aberrant genera and

1. Trans. Roy. Soc. Edin., vol. li, pt. iv.

2.

The type specimen of H. vespertilio (Murch. Sil. System, pl. xx, fig. ii) is from the basal Caradocian grits of Corston near Clunbury; it is in the museum of the Geological Survey.

3. Vide Bancroft "On the Notational Representation of the Rib-system in Orthacea." Mem, Manchester Lit, and Phil. Soc. lxxii, 1928,