256 DISCOVERY REPORTS 



literature on the genus; it has not been attempted here as these species are relatively 

 unimportant members of the skin film. 



Perhaps the strongest argument for regarding these " other species ", not known from 

 the littoral plankton, as true constituents of the skin film lies in the almost complete 

 absence of oceanic plankton forms occurring accidentally in the film, though they have 

 been noted on the baleen. If appreciable numbers of diatoms become involved in the 

 film while the whale is lying flagged at sea and being towed into harbour tail first, one 

 would expect to find oceanic plankton forms present. South Georgia is a small island, 

 and the characteristically scanty neritic plankton is confined to the bays and a very 

 narrow coastal belt. Offshore a very rich oceanic plankton is to be found during the first 

 half of the season, and as the whales are frequently captured over a hundred miles from 

 land, the force of this argument will be readily apparent. It seems certain that the 

 period while the whales are lying alongside the plan waiting to be flensed is the only 

 time at which contamination is likely to occur. For the reasons previously given, it is 

 extremely improbable that this ever takes place with regard to the species held to be 

 true constituents of the skin film. On the other hand, the species referred to as occurring 

 fortuitously upon whales include the dominant members of the neritic phytoplankton, 

 which very probably gain access to the whales while they are lying waiting to be flensed. 



NOTES ON THE SPECIES 



Lycmophora lyngbyei, Kiitz. This common littoral form, found in both hemispheres, 

 is so well known, and figured in so many works on diatoms, that detailed comment is 

 unnecessary here. An account of its occurrence upon whales at South Georgia has 

 already been given on p. 254. The majority of the frustules found in the skin film ap- 

 peared to be of the typical proportions described and figured by Lebour (1930, p. 203), but 

 a few specimens with more elongated valves were observed. It was noted that the colour 

 of the endochrome of this species varied from light yellowish green to dark yellowish 

 brown in similar fashion to that of the Cocconeis ceticola among which it was found. 



Cocconeis ceticola, Nelson (Plate XI, figs. 1-4). The form of the valves of this 

 species has been fully described by Nelson, its founder, in his appendix to Bennett's 

 paper (1920, p. 355). It may be mentioned that in making ordinary mounts of these 

 diatoms it was found to be very difficult to persuade them to lie with their superior 

 valves uppermost. This is apparently due to the lens-shaped character of the frustules 

 shown in Plate XI, fig. 2, in which an attempt has been made to depict the girdle view. 



In all the samples examined the vast majority of the individuals were within the size 

 limits laid down by Nelson in his original description of the species. It was noted, how- 

 ever, that there was a strong tendency for most of the diatoms on any given whale to 

 be very similar in size. Accordingly a close watch was kept upon the seasonal variation 

 in size, in the hope that some light might be thrown upon the life history of the diatoms. 

 They were roughly divided into large, medium and small size groups, and the dominant 

 group upon each individual whale recorded. Equal preponderance of two such size 

 groups appeared to be a very rare occurrence, except towards the end of the season. 



