GENERAL NOTES 215 



The Soleniineae are sharply defined from all other groups of diatoms both as regard 

 geological age, distribution, structure, and habit. Bacteriastrwn must be regarded as 

 having a tropical to temperate distribution and is but seldom found in cold waters. 

 It occurred frequently in plankton around South Africa. Much confusion exists con- 

 cerning the various species and their geographical limits. Pavillard (1924-5) did much 

 to dispel this, but from the material I have examined I am of the opinion that much 

 variation takes place within the several species and that the lines of demarcation be- 

 tween some of them at least are very vague. 



These weakly siliceous forms are so readily moulded by the varying environmental 

 conditions as to make specific identification extremely difficult. Fortunately the species 

 recorded from the ' Discovery' material were in most cases fairly definite, although in- 

 determinate forms appeared from time to time. B. elongatum and varieties of B. 

 hyalinum occurred most frequently, chiefly off the coast of South Africa. B. comosum 

 occurred high up off the east coast of Africa in the equatorial zone, and this species is 

 fairly common in the Mediterranean. 



The effect of environment on the organism is best seen in the next family, namely, 

 Corethronaceae. The type genus Corethron provides an opportunity for such a study, 

 and in order to avoid the creation of a host of ill-founded species, must be approached 

 with sanity and restraint. The genus has always been a difficulty, chiefly due to the 

 scarcity of material or rather that material is available only upon infrequent occasions. 

 Little or no account has been taken of the fact that Corethron is a genus possessing a 

 wide geographical distribution and that its members are subjected to considerable 

 variation in all the important factors, such as temperature, salinity, hydrogen-ion con- 

 centration, available oxygen, dissolved mineral salts, and sunlight, that control a plank- 

 tonic existence. These variations are not only observed in different places, but also in 

 the same place at different seasons of the year, or during different years, and it is reason- 

 able to suppose that such changes produce profound effects upon the organisms. 



The weakly siliceous frustules of Corethron, which bear few or no markings such as 

 the granules or costae that are so characteristic of diatoms, become plastic material ad- 

 mirably suited to abnormal development along the principal axis, which is one of the 

 chief characters of the suborder to which the genus belongs. As a population we are 

 dealing with weakly siliceous organisms distributed throughout the world and accom- 

 modating themselves to every possible variation of environment. 



The individuals vary considerably in shape and dimensions, some being almost 

 spherical ; but the majority of them are tubular, with the length of the frustule anything 

 from two to fourteen times the breadth. Some of the individuals are furnished with long 

 bristles at both ends of the frustule, others have bristles at one end only ; some of the 

 bristles bear small spines, others do not; frequently the bristles at one end of the 

 frustule bear spines, while those at the other end are plain. The frustules often bear 

 similar markings to those on Rhizosolenia. Other individuals are furnished with a 

 corona of fine bristles bearing claw-like extremities. 



Generally speaking these variants inhabit fairly well-defined areas, and their cha- 



