[bailey-mackay] diatoms FROM VANCOUVER ISLAND, B. C. 155 



admirably adapted for flotation, tends to form tangled masses from 

 which other forms are removed with difiiculty. Skeletonema though 

 apparently represented only by a single species, is also very abundant, 

 forming long chains or dense masses, sometimes indeed making up 

 nearly the whole of a particular gathering. Next to this species would 

 probably stand, Thalassiosira with its several species and the curious 

 connexion of its frustules by what the Germans call the slime-thread. 

 Seasonal Variations. The abundance and variety of the Plankton 

 vary greatly with the seasons, though no systematic observations 

 bearing upon the point have yet been made. Gatherings made in 

 early spring were in some instances almost destitute of Diatoms of 

 any kind, Thalassiosira being the most persistent, where a few months 

 later they were abundant. 



III. Non-Planktonic Forms. 



Habitat. The distinction between Planktonic and non-Plank- 

 tonic forms is not an absolute one. While species which like Chae- 

 toceros, Corethron, Thalassiosira and Skeletonema are rarely found in 

 shallow coastal waters, species proper to the latter may be drifted 

 from their native habitat by currents and thus become commingled 

 with those whose natural home is more distant from the shore and 

 which are more truly planktonic. Apart from habitat the most im- 

 portant distinction is to be found in the absence among the shore 

 forms of marked adaptations to flotation such as are found in the 

 presence of prominent horns or processes like those characteristic 

 of Chaetoceros, Skeletonema, Biddulphia, etc. These are rarely met with 

 in littoral species. 



On the other hand, while circular forms like Coscinodiscus are 

 abundant in the true plankton, other and related genera, such as 

 Actinocyclus, Actinopytchus, Aulacodiscus , Arachnodiscus, Cyclotella 

 are equally abundant nearer shore and are often found attached to or 

 sessile upon the fronds of marine algae. Such forms as Achnanthes, 

 Cocconeis, Gomphonema, Fragillaria, Grammatophora, Licmophora, 

 Melosira, Navicula, Pleurosigma, Rhabdonema, Synedra, and Surirella 

 are especially abundant in littoral gatherings. Some species of Cocco- 

 neis cover the flat surfaces of alga& in countless thousands. 



More Common Species. Among forms which are very common the 

 following m.ay be mentioned: 



Acnanthes longipes. 

 A. brevipes. 



Actinoptychus undulatus. 

 Biddulphia aurita. 



