206 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



The above table brings out clearly the marked 

 Diatom maximum in May, the minimum in July and 

 August when very few Diatoms were present, and the 

 second or autumnal maximum in October. The vernal 

 and autumnal maxima are shown unusually clearly by 

 Thalassiosira, which was present only at these periods, 

 being totally absent from November to the end of April, 

 and in July, August and September. Placed in the order 

 of their highest monthly averages throughout the year, 

 these common Diatoms are as follows :- — 



March — Coscinodiscus . 



May — Chaetoceras, Thalassiosira and Lauderia. 

 June — Rhizosolenia and Guinardia. 

 November — Biddulphia. 



On the whole, the regularity of occurrence and of 

 waxing and waning throughout the year, rather than 

 differences from year to year, is what strikes the observer 

 as of primary importance. 



Biddulphia mobiliensis and 

 B. sinensis. 

 Both the species of Biddulphia which we have been 

 obtaining in quantity during recent years (B. mohiliensis 

 and B. sinensis) occurred in October this year, and even 

 occasionally in September, an unusually early appearance 

 for B. sinensis. Fig. 1 shows a typical example of a 

 plankton containing abundance of both species, the 

 longer and relatively narrower forms in the figure being 

 B. sinensis, and the shorter, nearly square forms, 

 B. mobiliensis : there are, of course, other differences 

 which are not seen clearly in that figure. In the more 

 enlarged micro-photograph (fig. 2), a points to a typical 

 B. mobiliensis as seen in our district, and b to an 



