GENERAL NOTES 213 



from earlier deposits are particularly coarse and ill-formed, and their markings and 

 orientation might be described as primitive. The Oamaru deposit, however, contains 

 not only the primitive forms found in the earlier deposits, but examples of great evolu- 

 tionary progress, manifest in the regular orientation of the valve surface and a laminated 

 structure of the valve itself, as found in the arcticum and favus groups. These structural 

 innovations proved to be advantageous, for the really coarse granular forms have become 

 entirely extinct, and the members of the arcticum and favus groups enjoy a world-wide 

 distribution. The earliest deposit to contain diatoms is that at Kusnetzk, in South Russia, 

 where a great number of coarse species of Triceratium have been found. Only 2 per 

 cent of the total number of diatoms of that deposit are to be found living at the present 

 day — not one of the species of Triceratium has survived. If the main fossil marine de- 

 posits of the world are arranged in chronological order it will be found that the propor- 

 tion of the. favus group to the arcticum group varies with the temperature. Deposits laid 

 down in tropical seas have a larger proportion of favus forms than those laid down in 

 polar seas. This division is well marked in the forms found living at the present day. 

 The favus group has a temperate to tropical distribution, while the arcticum group is 

 subpolar. 



These two groups that have established themselves throughout wide yet definite areas, 

 and exhibit such a highly evolved structural development demand due recognition. 

 In the absence of experimental taxonomy, the important considerations enumerated 

 above, namely, characteristic structure, progressive development and geographical dis- 

 tribution, warrant the two groups being given generic status. The favus group has 

 already been dealt with, and the recognition of Cleve's genus Trigonium, which has 

 T. arcticum Cleve, based on Triceratium arcticum Brightwell, as the type species, 

 satisfies all the demands of the so-called arcticum group. 



Considering the number of inshore stations that have been examined, the number of 

 species of Biddulphia and Triceratium observed was surprisingly few. 



EUCAMPIOIDEAE. 



Coming to the suborder Eucampioidae, the weak silicification of the valves is very 

 pronounced and the plants have entirely changed their habits from solitary forms to 

 colonial. Long ribbon-like filaments of all the genera contained in the suborder are very 

 frequent in the plankton, and some of them must be considered truly oceanic. Eucampia 

 balaustium, for instance, is very common in cold water, and undergoes considerable 

 variation in size and degree of silicification. Generally speaking the oceanic forms are 

 more weakly siliceous than the neritic ones. 



In the genera Climacodium and Streptotheca silicification is reduced to the minimum, 

 and the latter, which is represented by one species only, can in no way be regarded as a 

 siliceous organism in the same way, for instance, that one would regard a Triceratium. 

 The frustules are quite soft and pectinous and absorb dyes readily. At this, the lower 

 end of the Biddulphiaceae, the term biddulphioid is not applicable in its true sense, as 

 a description of the species. 



