214 DISCOVERY REPORTS 



of this group belong to the two families Fragilariaceae and Naviculaceae, with a few species of the 

 genus Nitzschia. These forms, which fall into the suborders Araphidineae and Biraphidineae of 

 Hendey's system, exhibit features that fit them for a free-floating existence (if the teleological expres- 

 sion may be forgiven). Such features are the development of catenary, branching or ribbon-forming 

 colonies ; attenuated form of the individual cells ; or the possession of cell-walls much slighter than 

 those of their bottom-dwelling relatives. Indeed, in an aetiological sense, they have collectively been 

 well described as 'reversionary plankton forms', by Lloyd (1926). However, in this material, as in 

 that from other coastal areas, we find that a small proportion of bottom-dwelling forms (e.g. Achnanthes 

 which falls within Hendey's Monoraphidineae) occur tychopelagically in the plankton. In need of a 

 summary group-heading that would include these also, it was felt that the old term ' Pennatae ', though 

 of doubtful value in its taxonomic sense, would be more convenient than some cumbersome phrase 

 such as 'last five suborders'. The expression 'small pennate diatoms' that Marshall (1933) found 

 convenient for ecological description of material from the Great Barrier Reef, is only a partial equiva- 

 lent, since large species like Thalassiothrix longissima and Pleurosigma capense are included in the 

 Benguela grouping. 



For the rest, Hendey's suborder Discineae suited our purpose well as it stands, but coming to the 

 suborder Biddulphineae it seemed best to divide the outstandingly abundant Chaetocerids from the 

 others at family level, for the latter derive from several subfamilies, whereas the Chaetoceraceae 

 include but the one subfamily, and indeed, in this material, only the one genus. The Soleniineae, 

 consisting almost exclusively of holoplanktonic forms, provided a convenient grouping at the level of 

 the suborder. 



Thus four of our five arbitrary groupings can be defined in terms of widely accepted system- 

 atic assemblages, though these are not of equal status. The fifth group remains heterogeneous. 

 Abstracting them from the classification table, and summarizing their main ecological characteristics, 

 we have: 



I Suborder Discineae. Most of the species neritic and probably meroplanktonic, with heavily 

 silicified frustules. Exceptions (holoplanktonic, relatively more abundant in the sparser phyto- 

 plankton at greater distances from land) were some of the Thalassiosira spp. and the definitely oceanic 

 Planktoniella. The colonial habit and gelatinous or spiny projections of the former, and thin, hyaline, 

 ribbed extensions of the valve margins of the latter, may conceivably assist flotation. 



II Family Biddulphiaceae. Mainly neritic, few solitary, but the others showing very varied 

 development of the colonial habit. A few are oceanic, holoplanktonic (very rare in these samples, 

 e.g. Hemiaulus Hauckii). Majority meroplanktonic (Eucampia, Biddalphia) or even bottom-dwelling 

 species (Triceratium) whose occasional presence alive in the plankton may justify their inclusion as 

 tychopelagic species. 



III Family Chaetoceraceae. Mainly holoplanktonic in the vegetative phases, but a majority form 

 specialized resting spores and are, therefore, probably meroplanktonic. Many neritic, few oceanic 

 species. In comparison with II, with which they form the suborder Biddulphiineae, the Chaeto- 

 ceraceae show an increased development of structural features that must tend to aid flotation : thin, less 

 strongly silicified frustules, and horns produced into long setae. Mainly colonial. 



IV Suborder Soleniineae. Nearly all holoplanktonic, many oceanic or panthalassic, few neritic. 

 Structurally they include large and medium-sized tubular species, solitary or in colonies of few cells 

 and long chains of small cylindrical species. 



V ' Pennatae '. A heterogeneous arbitrary grouping, as explained in detail above. It consists 

 of the few normally planktonic members of Hendey's Araphidineae and Biraphidineae (the ' reversionary 

 plankton forms ' of Lloyd) with such few bottom-dwelling forms from the last five of Hendey's sub- 



