SYSTEMATIC ACCOUNT 289 



establish a chain of intermediate forms that would allow one to unite them under one 

 name were unsuccessful, in spite of the similarity of structure. 



The punctation upon the valve surface was very much finer than in Anaulus scalaris, 

 and in some specimens appeared to be absent altogether from some of the loculi. The 

 plicate connective zone was strongly developed and very much deeper than in Ehren- 

 berg's species. In the specimens examined the photosynthetic elements were indis- 

 cernible. The cells were united at one angle only by a short mucous thread or cushion, 

 forming small spirally arranged filaments. 



Observed at St. WS481. 



Anaulus scalaris Ehrenberg. 



In H. van Heurck, 1909, p. 35, pi. 8, figs. 108, 109. 



Cells elongated, very robust. Valves narrowly elliptical with broadly rounded apices. 

 The extremities of the valve are furnished with large ocelli, shaped somewhat like a 

 thumb-mark. Ocelli only very slightly raised above the surface of the valve. Valve sur- 

 face almost flat, furnished with numerous transverse bars, usually eight to twenty, which 

 divide the valve into almost equal compartments, those towards the apices usually a 

 little smaller than the central ones. These compartments are covered with a very fine 

 striation, which has a tendency to subradial arrangement. Cell in girdle view rect- 

 angular, connective zone simple. Apical axis of cell 200-300/x ; transapical axis 26-40/j.. 



Observed at St. WS481. 



Family CHAETOCERACEAE 

 Subfamily CHAETOCEROIDEAE 



1. Cells bipolar, hyaline, each angle furnished with one long bristle ... ... Chaetoceros 



Genus Chaetoceros Ehrenberg 

 Ehrenberg, 1844 



Chaetoceros is the largest of the truly planktonic genera, containing approximately 

 160 species. The majority of them are neritic, and the classification depends upon the 

 form of the cell, the number and position of the chromatophores, the form of the 

 foramina or intercellular apertures, and the structure of the characteristic appendages. 

 Great confusion has been caused because of the difference in appearance exhibited by 

 the same species from different localities, and at different seasons of the year. The pro- 

 duction and identity of so-called winter forms and summer forms is very difficult to 

 appreciate unless intermediates are available to complete the gradation — so different are 

 the extremes that they may be taken for totally different species. 



Members of the genus may be solitary, but usually they are colonial, and are to be 

 found matted together in very dense populations. Some small species form mucilaginous 

 colonies, particularly in cold waters. The genus is spread equally through warm and 



