184 PROF. W. A. HERDMAN 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 Chetoceras throughout 
many of the months*. There are no months in the ten years when Chetoceras 
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; 12} 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 Chetoceras, to form the main crest on the vernal 
Diatom curve (see fig. 5). 
Fic. 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. Nordenskioldi, Cleve (fig. 8). Apparently 
T. gravida is the only one common at Plymouth, but 7. Nordenskioldi, along 
with Chetoceras contortum and C. debile, 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 Chetoceras teres, C. debile, aud 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). 
