486 Mr. C. Mereschkowsky on 



the first {B. {T.) alter nans) is very frequent, has shown me 

 without any doubt that they cannot be considered two 

 different species, being connected one with tlie other by all 

 possible intermediate forms. Usually the B. {T.) alternans 

 has regularly triangular valves, with the three apices of equal 

 length ; but there occur forms in which, while one or two 

 apices are broadly rounded, as in the type species, the other 

 one (or sometimes two) is acute and has the peculiar irregular 

 appearance which characterizes the varietas variahilis. 



In figs. 9, 10, 1 have represented some of these interme- 

 diate forms. As to the structure of the valve, it is identical in 

 bothj and their size is also the same. 



B. (T.) alternans being the more common form, I regard 

 it as the type species and the other as its variety. 



24. Cyclotella striata (Kiitz.), Grun. Rare. 

 Diameter from 0*0185 to 0"031 mm. 



25. Auliscus sculptns, var. coelata, Bail. Rare. 



The only specimen observed agreed pretty well with the 

 figure of Bailey. 



26. Actinoptychus undulatus^ Ehr. Not very rare. 



Often with a small process placed on the submarginal 

 median portion of each alternate compartment, sometimes of 

 all compartments. 



27. Actinoptychus areolatus, Brightw. Rather rare. 

 Seems to be only a variety of the preceding species. 



28. Actinoptychus splendens (Schadb.), Ralfs. Rare. 



29. Asteromphalus flahellatna (Breb.), Grev. Not very rare ; 

 a widely distributed species. 



30. Asteromphalus heptactis (Brdh.), Ralfs (Pritchard, Inf. 

 pi. viii. fig. 21 ; SiJatangidium Ralfsianum, Norm. Micr. 

 Journ. vii. (1859), pi. vii. figs. 7, 8; Asteromphalus Ralfs- 

 ianus, A. Schmidt, Atlas, pi. xxxviii. figs. 5-8) *. 

 Rather rare. 



♦ The above-cited synonyms are given after Cleve, " Report on the 

 Phyto-Plankton collected on the Expedition of H.M.S. ' Research,' 1896," 

 Fifteenth Annual Report of the Fishery Board for Scotland, part iii. 

 p. 297. 



