PLAyKTOX DIATOMS IX .ST. ANDREWS WATERS 53 



onwards the remaining- forms appear. The gi-eat predominance of Ch. dehile, which 

 on August 2 gives the record count of 7,000,000 frustules, is to be noted. The graceful 

 spiral chains of this species are a characteristic feature of summer gatherings. But 

 the maximum for diversity of form is, as recorded above, in September. 



The allied genera, Corethron, Ditylium and Rhizosolenia, also attain their 

 maxima vn. the autumn. Co/rethrdn criophihim appears only occasionally; but the 

 beautifully modelled Ditylium Brightwelii is a dominant plankton form from the end 

 of September until the first of December. In the autumn four species of Rhizoso- 

 lenia ure abundant, but throughout the winter and until the following August only 

 R. hebitata is found. McMurrich (13) has recorded a distinct spring maximum for 

 B. setigera in 1'915, but this was iiot repeated in 1917. 



Another dominant autumn form is Thalassiothrix longissima, which attains a 

 sudden maximum in October, but holds its position of prominence for but a brief 

 period. Its allied species T. nitzschioides is present in varying, but never great num- 

 bers throughout the year. 



A prevalence of free living, compact forms is to be -noted in winter. Pleurosigma, 

 but scantily represented during the autumn, presents six species in February. The 

 only one, however, which can be said to be characteristic of any season is P. strigosum, 

 which abounds from February until April. December brings in Rhabdonema and 

 Surirella, and January the Campylodisci ; Actinopiyclius undulatus and the Coscino- 

 disci persist and the latter presents an increase in the number of species. The 

 majority of the more delicate forms, Leptocylindrus, Cerataulina, etc., fail; but 

 filamentous forms are not entirely lacking, for Skeletonema costaUtm aiid Melosira 

 are taken in practically every collection. 



The prevailing spring forms are Biddulphia and Tlialassiosira. The former is 

 introduced in December and occurs in small numbers during the winter. It then 

 gradually increases and attains a distinct maximum in the middle of March, after 

 wliioh its numbers decrease; and it is rarely found after May. For B. sinoisis a simi- 

 lar maximum has been recorded by Ostenfeld (16) in the Korth sea, but it there 

 prevailed throughout the summer and reached its height in November. Thalassiosira 

 appears in February. Five species T. gravida, nordensl'ioldii, hyalina, condensata 

 and Coscinosira poly.choi-da, are grouped together in the tables. These dominate the 

 plankton during April and May and on May 1 give the enormous total of 8,750,000 

 frustules. 



It is seen that in general the autumn plankton is characterized by the presence 

 of slender, elongated forms such as Thalassiothrix and Rhizosolenia, together with 

 numerous species of Chaetoceras. The winter presents the solid, compact forms, 

 vv'hile in spring and summer the long, graceful chains of Thalassiosira and Chaeto- 

 ceras ]3revail. Other species appear occasionally, or are present in small numbers 

 throughout the y^ar, but at no time does any other form a characteristic, seasonal 

 feature. 



