2IO DISCOVERY REPORTS 



The genus Charcotia of Peragallo has been adopted on account of the dissimilarity of 

 the valvar markings and the localization of its distribution. These organisms are very 

 characteristic, and although they should be regarded as bottom forms are to be found 

 occasionally in the plankton. 



The levigated discoids are a peculiarly interesting group and might be regarded as 

 being derived from bottom forms. They may have adapted themselves to a planktonic 

 existence by the development of certain structures that increase buoyancy and so enable 

 heavy frustules to overcome the decrease in density of the surface layers of the ocean. 

 This is accomplished by the production either of coronas of spines or circumferential 

 loculi which may be filled with gases or liquids of low density. In Gossleriella the former 

 method is used. Considerable variation in the degree of spininess was observed. The 

 valve may develop one, two or even three rows of spines around its circumference, and 

 in some specimens a group of spines was observed springing from the centre of the valve 

 face. This suggests that valve-to-valve linking might take place in an attempt to adopt 

 colonial habits. In Planktoniella the second method is adopted. The valve proper is 

 stout, and as a rule strongly siliceous, and is furnished with a peripheral wing-like 

 expansion, divided by radial rays, producing a number of chambers. These chambers 

 may have the property of becoming turgid or flaccid according to the requirements of 

 the organism. By the regulation of the turgidity of the peripheral chambers it is likely 

 that the organism can change its habit from a bottom form to that of a true plankton 

 form as occasion demands. 



The family Actinodiscaceae might be conveniently divided into oceanic forms and 

 neritic forms. In the first class are Asterolampra and Aster omphalus. 



Asterolampra is a temperate to tropical genus favouring a rather high salinity. A small 

 form of A. marylandica was frequently observed in the Atlantic between the Equator 

 and the latitude of 40 S. This form possessed seven rays and was remarkably constant 

 in diameter. A specimen possessing eight rays was also observed near the Equator. The 

 variation in the number of rays is of no specific importance, as this is controlled by the 

 internal pressure of the frustule. In the genus Asteromphahis the habit is truly oceanic, 

 and the species are widespread in the sub-Antarctic Zone, although they never appear 

 in great numbers. A. parvulus, a small form, was found very far south in the Weddell 



Sea. 



The genera Stictodiscus and Arachnoidiscus are littoral, although the former is some- 

 times observed in deep-sea plankton. The occurrence of A. Ehrenbergii in the cold 

 waters of the Bransfield Strait was rather surprising, as this species is usually confined 

 to subequatorial regions. The frustules were large and vigorously developed and ap- 

 peared to differ in no way from the specimens obtained from warm water. It is a com- 

 plete mystery how this species became established in so cold a habitat. A careful ex- 

 amination of all the plankton from a long line of inshore stations along the Chili-Peru 

 coast did not reveal any trace of it. The species is frequently found off the coasts of 

 Japan and California, but it is almost inconceivable that the Bransfield colony originated 

 from California, as there is no surface current which would account for this extension 



