10(3 PROF. W. A. HERMAN ON THE DISTRIBUTION 



The highest average is nearly 23 millions, in May 1911. None of the 

 September and October averages run as high as those in spring, and only two 

 reach millions, viz., 3,956,047 in October 1911, and 7,702,658 in September 

 1912. The years 1911 and 1912 had high numbers of Cluetoceras throughout 

 many of the months*. There are no months in the ten years when Chcetoceras 

 was totally unrepresented ; but July and August show the lowest averages' — 

 the lowest of all being only six individual cells in August 1907. 



Lauderia. 



We have only the one species, Lauderia borealis, Gran (fig. 7), in our 

 records. It is a late spring or early summer form, occurring generally from 

 March or April to June or July, with a later, smaller, occurrence in autumn. 

 It is sometimes present in large quantities, e. g., 20,064,000 on April 22nd, 

 1910; 121 millions on April 29th, 1912; 3,600,000 on May 4th, 1914. 

 The maximum is towards the end of April or beginning of May, when 

 Lauderia helps, along with Chcetoceras, to form the main crest on the vernal 

 Diatom curve (see fig. 5). 



Fig. 7. — Photo-micrograph showing a chain of Lauderia borealis. 



Thalassiosira. 



The only species of this genus that are of any importance in our records 

 are T. gravida, Cleve, and T. Nordenshioldi, Cleve (fig. 8). Apparently 

 T. gravida is the only one common at Plymouth, but T. Nordenshioldi, along 

 with Cluetoceras contortum and C. dehile, helped to constitute the vernal 

 maximum at Port Erin in 1907, and has been still more abundant on several 

 occasions since. T. Nordenskioldi is in the main a neritic, arctic or 

 Scandinavian species, and probably its occasional occurrences in quantity 

 are to be regarded as invasions of some arctic water and northern plankton 

 into our British seas. In April 1917, it was abundant at Port Erin along 

 with Cluetoceras teres, C. dehile, and C. decipiens. 



All our high records (over a million per haul) for Thalassiosira lie between 

 late in April and late in May, and the two highest are six millions on April 

 29th, 1912, and six and a half millions on May 16th, 1913. Other high 



* The largest hauls of Diatoms as a whole, all species taken together, were in May of 

 1912 and 1913 (see Table on p. 188). 



