152 SIR W. A. HERDMAN : RESULTS OF CONTINUOUS 
Chætoceras. 
This is the most abundant and characteristic form of the spring (April and 
May) plankton in the Irish Sea. It isa large genus containing many species, 
of which at least a dozen have occurred in our gatherings off Port Erin, but 
it is only the following eight that attain to such numbers as to be really 
dominant constituents of the plankton, viz.: C. boreale, C. contortum, 
C. eriophilum, C. debile, C. decipiens, C. densum, C. sociale, and C. teres. Of 
these, again, the most abundant species are C. debile, C. decipiens, C. sociale, 
and C. teres. 
Some years seem to be much more favourable for the development of vast 
numbers of diatoms than others, some are good years for Chætoceras but not 
for Rhizosolenia, and some are characterised by great abundance of one 
particular species. For example, 1907 was a poor year for both Chetoceras 
and Rhizosolenia, 1912 was a good year for both, 1917 was a poor year for 
Ithizosolenia, but showed Chetoceras in abundance. Again, Chetoceras 
sociale, usually one of the most abundant species, was very poorly represented 
in 1918 and was practically absent in 1920, and C. contortum, present and 
prominent in most years up to 1915, was very scanty in 1918 and absent in 
1919 and 1920. Consequently, from our statistics, we can speak not only of 
a “ Chætoceras ” year and “ Rhizosolenia ” year, but also of 1909 being a 
“teres ” year and of 1910 being a * debile ” year, and so on—as the following 
statement will show in detail :— 
Prevalence of Chetoceras during 15 years *. 
1907 —A poor year, no millions per haul present at any time. 
1908—-A pril, a fair amount but no millions; May, C. boreale up to 1 million. 
1909— April, C. teres up to 8 million. 
1910—A pril, C. debile up to 24 mill. (on 22nd) ; May, C. sociale up to 15 mill. 
1911— May, C. debile (30 mill.) and C. sociale ; Sept., C. debile, decipiens, and teres. 
1912—April, abundance of C. sociale (44 mill), debile (36 mill), decipiens and feres 
(16 mill., in March) ; Sept., C. decipiens (25 mill.). . 
1913—May, C. debile, but less than 1912. 
1914—May, C. debile in large quantity (over 100 mill.). 
1915—May, C. debile, criophilum, and sociale abundant, 
1916—May, C. sociale (up to 93 mill.). 
1917— April, abundance of C. debile (over 44 mill.) and teres (16 mill.); May, debile and 
sociale (30 mill.). 
1918—May, C. debile (up to 38 mill.). 
1919—A pril, C. decipiens ; May, debile. 
1920—May, C. debile and decipiens, 
1921— Late April and early May, C. debile, decipiens, and teres. 
Some of the species of Chetoceras (debile, decipiens, densum, contortum, 
eriophilum, and teres) begin to appear in small quantities quite early in the 
* The months named are those in which diatoms of the genus Chetoceras were most 
abundant in each of the 15 years; and when specific names are given they are the species 
which were most prominent in the plankton, and reached millions per haul at that time. 
